Physics

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772 domain-specific abstractions whose origin domain is Physics. They span 613 subdomains — sort by that column to group them, or click any subdomain to filter to it.

Abstraction	Subdomain	Description
A Posteriori Necessity	MetaphysicsModality and Reference	A proposition has a posteriori necessity when its truth requires empirical warrant while its modal status is necessary: once the actual referents, identities, or essences are fixed, the proposition could not have been false.
Abbe Number	Optical Materials	Compare a transparent medium's central-line refractivity with its short-to-long-line refractive-index difference under a declared spectral convention.
Absolute horizon	General Relativity	In general relativity, an absolute horizon is a boundary in spacetime, defined with respect to the external universe, inside which events cannot affect an external observer.
Absorbed Dose	Dosimetry	Absorbed dose is the mean energy imparted by ionizing radiation per unit mass of a specified absorber, measured in gray.
Absorption (acoustics)	Acoustics and Building Physics	The conversion or transfer of incident sound energy within a material or structure so that less acoustic energy is reflected back into the originating medium.
Accidental symmetry	Effective Field Theory	A symmetry of the low-energy or restricted effective dynamics that is not imposed fundamentally but appears because all symmetry-violating operators are absent, forbidden at low dimension, or irrelevant.
Acoustic Shadow	AcousticsWave Propagation	An acoustic shadow or sound shadow is an area through which sound waves fail to propagate, due to topographical obstructions or disruption of the waves via phenomena such as wind currents, buildings, or sound barriers.
Acoustic streaming	Fluid Dynamics	A steady fluid flow generated when attenuation or boundary dissipation converts oscillatory acoustic momentum into a time-averaged body force or stress.
Acoustic transmission	Acoustics	Acoustic transmission is the transmission of sounds through and between materials, including air, wall, and musical instruments.
Acoustic wave	AcousticsMechanical Wave Propagation	Propagate a mechanical disturbance through a material medium as coupled variations of pressure, stress, density, and particle motion, with wave type fixed by the medium and restoring response.
Acoustoelastic Effect	AcousticsElasticityNondestructive Evaluation	A prior static stress or deformation changes how an incremental elastic wave propagates through a solid, with a response that depends on material, wave mode, and path.
Action (physics)	Classical MechanicsLagrangian Mechanics	In physics, action is a scalar quantity that describes how the balance of kinetic versus potential energy of a physical system changes with trajectory.
Active Brownian Particle	Active Matter	A stochastic particle model combining persistent self-propulsion with translational and rotational fluctuations, so nonequilibrium motion emerges without an externally imposed directional force.
Adiabatic invariant	Dynamical Physics	A quantity that remains approximately constant while a system's parameters vary sufficiently slowly relative to its intrinsic dynamics.
Adiabatic Process	—	A thermodynamic process constrained so that no heat crosses the chosen system boundary, while work, matter flow, internal dissipation, and state change remain separately accountable.
AdS/CMT Correspondence	Theoretical PhysicsHolographic Condensed Matter	Model selected observables of a strongly coupled condensed-matter quantum field theory with a tractable higher-dimensional gravitational system, using an explicit bulk-boundary dictionary and carrying predictions back only within the holographic model's declared validity envelope.
Afocal system	OpticsGeometrical Optical Systems	An optical system with zero net vergence and effectively infinite focal length, mapping collimated input rays to collimated output rays while changing angle or beam diameter.
Aharonov–Casher effect	Quantum Interference and Electromagnetic PhaseQuantum Mechanics	A magnetic-moment carrier acquiring a path-dependent quantum phase from an electric-field configuration.
AKLT Model	—	A frustration-free quantum antiferromagnet whose local higher-spin projectors make a valence-bond-solid state an exact ground state with a gapped one-dimensional prototype and fractionalized boundary spins.
Alfvén Wave	Plasma Physics	Propagate a low-frequency transverse disturbance through magnetized plasma as magnetic-field-line tension restores the displacement and plasma mass supplies inertia, with ideal speed set by field strength over the square root of mass density.
Algorithmic Cooling	Quantum Information ScienceQuantum Thermodynamics and Spin Ensemble Computation	Concentrate entropy away from selected qubits through reversible population compression and, in heat-bath variants, repeatedly refresh designated reset qubits so entropy leaves the working register, increasing target polarization or purity beyond closed-system compression limits.
Alpha Decay	Nuclear PhysicsCharged Particle Radioactivity	A nuclear decay channel in which an energetically open parent state becomes a daughter with A-4 and Z-2 while emitting a helium-4 nucleus.
Alpha-particle spectroscopy	Nuclear PhysicsRadiometric Analysis	Analyzing alpha-emission energy spectra to identify or estimate alpha-emitting radionuclides.
Amplituhedron	Mathematical Physics	A positive geometric object whose canonical differential form encodes scattering amplitudes in planar maximally supersymmetric Yang–Mills theory without making locality and unitarity manifest inputs.
Anderson Function	Magnetometry	Express the scalar projection of a magnetic dipole field sampled along a straight path as a linear combination of three dimensionless rational basis functions.
Angular displacement	Rotational Kinematics	The directed change in rotational position between two orientations about a specified axis or within a declared orientation representation.
Angular momentum operator	Quantum Mechanics	In quantum mechanics, the angular momentum operator is one of several related operators analogous to classical angular momentum.
Antiparticle	Particle Physics	In particle physics, every type of particle of "ordinary" matter (as opposed to antimatter) is associated with an antiparticle with the same mass but with opposite physical charges (such as electric charge).
Antisymmetrizer	Mathematical Physics	A linear projection that averages all particle-label permutations with their signs, extracting the totally antisymmetric component required for identical fermions.
Arbitrary unit	Scientific Measurement and Data Reporting	A locally defined relative measurement unit that reports magnitude against an instrument-, procedure- or experiment-specific reference rather than a universally standardized physical scale.
ARGUS distribution	Particle PhysicsExperimental High Energy Physics	A bounded probability density used to model smooth combinatorial background in reconstructed particle-candidate mass, with a sharp kinematic endpoint and a shape parameter controlling the fall toward that cutoff.
Askaryan Radiation	Astroparticle Physics	Coherent radio-frequency Cherenkov emission from the time-varying negative charge excess in a relativistic particle cascade propagating through a dielectric medium, strongest near the Cherenkov angle when wavelengths resolve the shower coherently.
Atmospheric refraction	Atmospheric OpticsWave Propagation	The bending of electromagnetic waves or sound through the atmosphere as refractive properties vary with altitude or local density and temperature, shifting, distorting, dispersing, or miraging apparent sources.
Atmospheric Window	Atmospheric PhysicsAtmospheric Radiative Transfer	A wavelength or frequency interval in which Earth's atmosphere has comparatively high electromagnetic transmittance, creating a path for radiation between the surface and space whose usefulness and boundaries depend on absorbers, scatterers, clouds, and path conditions.
Autowave	Nonlinear Dynamics	A self-sustaining nonlinear wave that propagates through an active medium supplied with distributed energy.
Axial Multipole Moments	ElectrostaticsMultipole Expansion	Axial multipole moments are a series expansion of the electric potential of a charge distribution localized close to the origin along one Cartesian axis, denoted here as the z-axis.
B92 protocol	Quantum Cryptography	A two-state quantum key-distribution protocol whose security relies on the impossibility of perfectly distinguishing nonorthogonal quantum states without disturbance.
Baade–Wesselink Method	Stellar AstrophysicsPulsating Star Distances	A pulsating-star distance method that fits phase-resolved angular-diameter variation to physical radius displacement inferred by integrating projection-corrected spectroscopic radial velocities.
Background-Selected Propagation Frame	AcousticsGravitational Theory	A local rest direction selected by a physical medium or modeled timelike field that enters a specified propagation law.
Backscattering cross section	Wave Scattering	An effective area quantifying the wave power an object returns toward the incident direction relative to incident intensity under a declared convention.
BB84	Quantum Cryptography and Quantum Communication	A quantum key-distribution protocol in which randomly chosen conjugate encoding bases let two parties sift correlated bits and estimate whether interception disturbed the channel before deriving a shorter secret key.
BCS theory	Condensed Matter PhysicsMicroscopic Theory of Superconductivity	Model conventional superconductivity microscopically by an effective attraction that pairs fermions near the Fermi surface into a phase-coherent many-body state with a self-consistent excitation gap, while separating the original weak-coupling isotropic theory from justified extensions.
Beam Propagation Method	Computational Optics	Approximate predominantly forward optical-wave evolution by factoring out a carrier, reducing the Helmholtz or Maxwell problem to a one-way propagation equation, and marching its transverse field through longitudinal steps.
Beam Waist	Optics and Laser Physics	The plane and radius at which a Gaussian beam is narrowest, fixing its Rayleigh range, divergence, and focusing geometry for a stated wavelength and medium.
Bell diagonal state	Quantum Information	Bell diagonal states are a class of bipartite qubit states that are frequently used in quantum information and quantum computation theory.
Bell's Spaceship Paradox	Special Relativity	A special-relativity thought experiment in which identically accelerated, equally separated spaceships make an initially taut connecting thread acquire increasing proper separation and break.
Berezinskii–Kosterlitz–Thouless Transition	Statistical MechanicsTwo Dimensional Systems	A two-dimensional phase transition in which a bound vortex-pair regime gives way to proliferation of free vortices and changed long-distance response.
Beta Function (Physics)	Theoretical PhysicsQuantum Field Theory	A renormalization-group vector field that gives each running coupling's logarithmic derivative with respect to scale, thereby generating scale flow and locating fixed points where the coupling ceases to run.
Bethe formula	Charged Particle TransportAtomic Physics	The Bethe formula gives the mean energy loss per path length of a fast charged particle traversing matter through ionization and excitation, as a function of charge, speed, and absorber properties.
Bethe–Feynman formula	Nuclear PhysicsNuclear Weapons Physics	The Bethe–Feynman formula is a historical theoretical relation for estimating the efficiency and energy yield of a fission explosive from bulk physical parameters.
BGK Collision Operator	Kinetic TheoryPlasma Physics	A kinetic collision model that relaxes a distribution toward a locally moment-matched Maxwellian at a specified collision rate.
Bickley Jet	Fluid MechanicsLaminar Free Shear Flows	The far-field self-similar solution for a steady two-dimensional laminar plane jet into quiescent fluid, with conserved momentum flux, entrainment, and characteristic downstream scaling.
Bidirectional Reflectance Distribution Function	OpticsRadiometry	Bidirectional Reflectance Distribution Function is a recurring optics, computer graphics, computer vision identity in which a four-variable function maps incoming and outgoing light directions to reflected radiance per incident irradiance at a surface.
Biexciton	Condensed Matter PhysicsSemiconductor Optics	Biexciton is a recurring identity in natural science, engineering, and health defined by this frozen evidence: In condensed matter physics, biexcitons are created from two free excitons, analogous to di-positronium in vacuum.
Binary collision approximation	Condensed Matter PhysicsIon Solid Interactions	In condensed-matter physics, the binary collision approximation (BCA) is a heuristic used to more efficiently simulate the penetration depth and defect production by energetic ions (with kinetic energies in the kilo-electronvolt (keV) range or higher) in solids.
Bjerknes Force	Bubble Acoustics	An acoustically pulsating bubble feels a cycle-averaged translational force when its volume change correlates with a spatial pressure gradient.
Black Hole	General RelativityBlack Hole Physics	A relativistic spacetime region whose event horizon prevents any future-directed signal or material trajectory inside it from reaching the exterior universe.
Black Hole No-Hair Theorem	General RelativityBlack Hole Uniqueness	A family of uniqueness results classifying stationary four-dimensional Einstein–Maxwell black-hole exteriors by mass, angular momentum, and charge under stated hypotheses.
Bogoliubov Quasiparticle	Condensed Matter PhysicsSuperconductivity	A canonical particle–hole excitation that diagonalizes a quadratic paired-fermion Hamiltonian, with normalized coherence factors and a pairing-dependent spectrum.
Bogomol'nyi–Prasad–Sommerfield state	Supersymmetric Field TheoryString Theory	where: \alpha \dot{\beta} are the Lorentz group indices, A and B are R-symmetry indices.
Bohr model	History of Atomic and Quantum Physics	A historical atomic model with electrons restricted to discrete stationary orbits and emitting or absorbing photons only when transitioning between quantized energy levels.
Bolometric Detection	RadiometryThermal Detectors	Bolometric detection reads incident radiation through the temperature change its absorbed power causes in a thermally referenced sensor.
Bose–Einstein condensation of polaritons	Semiconductor OpticsCondensed Matter Physics	Bose–Einstein condensation of polaritons is a growing field in semiconductor optics research, which exhibits spontaneous coherence similar to a laser, but through a different mechanism.
Bose–Einstein condensation of quasiparticles	Condensed Matter Physics	Macroscopic low-energy occupation of bosonic collective-excitation modes under system-specific population and relaxation conditions.
Brendel–Bormann oscillator model	Optical Material ModelsOptics	An optical dielectric-response model that Gaussian-broadens Lorentz oscillator resonances to represent non-Lorentzian absorption and dispersion, with analyticity and causality claims requiring care.
Brinkmann Coordinates	General Relativity	A wave-adapted coordinate system for pp-wave spacetimes that makes a parallel null direction explicit and puts the Lorentzian metric in Brinkmann form.
Brownian Dynamics	Soft Matter PhysicsStatistical Mechanics	Model physical particle configurations with overdamped force–mobility drift and matched thermal fluctuations.
C-energy	General RelativityCylindrical Gravitation	A quasi-local scalar measure of gravitational energy per unit Killing length in cylindrically symmetric spacetimes, designed to characterize the strength and flux of cylindrical gravitational fields.
Cabibbo–Kobayashi–Maskawa matrix	Particle Physics	A unitary matrix relating quark flavor eigenstates to weak-interaction eigenstates in the Standard Model.
Canonical commutation relation	Quantum MechanicsOperator Algebra	The quantum-mechanical operator relation between canonical conjugates, exemplified by [x,p]=iℏI, encoding their noncommutativity and fixing the associated uncertainty and representation structure.
Canonical Ensemble	Statistical Mechanics	Model equilibrium microstates at fixed temperature, volume and particle number by normalized Boltzmann weights while system energy can fluctuate.
Cardinal point (optics)	Geometrical OpticsGaussian Optics	One of the paired focal, principal, or nodal reference points that reduces a centered paraxial optical system to a small set of imaging relations.
Carnot's theorem (thermodynamics)	Thermodynamics	The thermodynamic result that no engine operating between two heat reservoirs can exceed the efficiency of a reversible engine between them, and all such reversible engines share efficiency 1−Tc/Th on an absolute temperature scale.
Cat state	Quantum MechanicsQuantum Optics	In quantum mechanics, the cat state, named after Schrödinger's cat, refers to a quantum state.
Cauchy elastic material	Continuum Mechanics	A simple elastic material whose current stress depends only on the current local deformation, not on its history or rate.
Cauchy's Equation	OpticsOptical Dispersion	An empirical optical dispersion formula expressing a transparent material's refractive index as A + B/λ² + C/λ⁴ + … over a fitted normal-dispersion wavelength range.
Causal Dynamical Triangulation	Quantum GravityLattice Approaches to Quantum Gravity	A nonperturbative lattice-gravity construction that sums Regge-weighted, causally admissible piecewise-flat spacetime histories and searches their phase structure for a continuum quantum geometry.
Cavity method	Statistical PhysicsDisordered Systems	A statistical-physics technique for disordered mean-field systems that removes one variable, characterizes the effective field from the remainder and imposes self-consistency when the variable is restored.
Center of percussion	Rigid Body Dynamics	The point on a pivoted rigid body where a perpendicular impulse produces no instantaneous reaction impulse at the pivot.
Central potential	Classical Mechanics	Central forces that are conservative can always be expressed as the negative gradient of a potential energy.
Centripetal Force	Classical Mechanics	The inward component of the net force required to bend a body's velocity along a curved path, equal in uniform circular motion to \(mv^2/r=mr\omega^2\) and supplied by ordinary interactions rather than by a separate force species.
Centrode	Kinematics	The locus traced by the instantaneous center of rotation of a rigid body undergoing planar motion.
CGHS model	Quantum GravityTwo Dimensional Dilaton Gravity	The Callan–Giddings–Harvey–Strominger (CGHS) model is a toy model of general relativity in 1 spatial and 1 time dimension.
Chandrasekhar Polarization	AstrophysicsStellar Atmospheres and Polarized Radiative Transfer	Solve polarized radiative transfer through a plane-parallel, scattering-dominated atmosphere to obtain a tangential center-to-limb polarization field that cancels on a symmetric unresolved disk but becomes observable when the projected weighting breaks that symmetry.
Chandrasekhar's white dwarf equation	AstrophysicsWhite Dwarf Structure and Degenerate Matter	Model a cold, completely degenerate white dwarf by a dimensionless nonlinear radial initial-value equation coupling electron-degeneracy pressure to Newtonian hydrostatic self-gravity.
Channel-Length Modulation	Semiconductor Device PhysicsField Effect Transistor Operation	The saturation-region field-effect-transistor phenomenon in which increasing drain bias moves the pinch-off boundary toward the source, shortening the effective channel and producing finite output resistance.
Chaotic mixing	Fluid Dynamics	Fluid mixing produced by repeated stretching and folding of material elements, generating exponentially fine filaments even in deterministic flows.
Chapman function	Atmospheric Physics	A dimensionless spherical-atmosphere integral giving slant-path column density relative to a vertical column for an exponentially stratified constituent.
Characteristic state function	Statistical MechanicsThermodynamics	The characteristic state function or Massieu's potential in statistical mechanics refers to a particular relationship between the partition function of an ensemble.
Charge number	Electromagnetism and Chemical Notation	The dimensionless electric charge of a particle, ion or system expressed as a signed multiple of the elementary charge, z=q/e.
Charge Qubit	—	A superconducting circuit qubit whose computational states are distinguished primarily by excess Cooper-pair charge on a small island.
Classical Electromagnetism	Electromagnetic Field Theory	A classical field theory in which charge and current source coupled electric and magnetic fields through Maxwell's equations, the fields act on charged matter through the Lorentz force, and initial, boundary, and material relations close predictions of force, radiation, energy, and momentum.
Classical mechanics	Theoretical and Applied Mechanics	A physical theory describing macroscopic motion through forces, mass, momentum, energy, and deterministic equations without quantum effects and usually without relativistic corrections.
Classification of Electromagnetic Fields	Theoretical PhysicsRelativistic Electromagnetism	A pointwise relativistic taxonomy that uses the two Lorentz scalar invariants of the electromagnetic field tensor to distinguish null and non-null fields and determine which electric–magnetic simplifications some observer can realize.
Clifford gate	Quantum InformationQuantum Computing	A quantum unitary that normalizes the n-qubit Pauli group, mapping every Pauli operator to another Pauli operator under conjugation.
Closed timelike curve	General Relativity	A future-directed timelike worldline in spacetime that closes on itself, returning an observer or particle to the same spacetime event.
Clustering of Self-Propelled Particles	Active Matter	The nonequilibrium formation of finite dynamic clusters or dense active phases when self-propulsion and interaction-dependent retention make motile particles accumulate faster than they escape or fragment.
Coefficient of Fractional Parentage	Angular Momentum CouplingAntisymmetric State Expansions	Resolve an antisymmetric many-fermion state into coupled one-particle-plus-parent channels using convention-dependent expansion amplitudes.
Coefficient of performance	Thermodynamics	The ratio of useful heating or cooling delivered by a heat-moving system to the external work or energy input required.
Coincidence counting (physics)	Experimental Quantum Physics	A measurement method that registers detections in separate channels as one event when their timestamps fall within a declared coincidence window.
Colloidal crystal	Soft Matter PhysicsColloidal Self Assembly	An ordered periodic or locally periodic array of colloidal particles whose lattice-scale spacing produces crystal-like mechanics, phase behavior and often optical diffraction.
Color Quality Scale	Colorimetry	Score how a test light renders a standardized set of high-chroma samples relative to a reference illuminant while treating fidelity, gamut change, and chromatic adaptation explicitly.
Color Solid	Color ScienceColor Space and Gamut Representation	Represent a declared set of surface or reproducible colors as a bounded three-dimensional region in specified color coordinates, exposing its gamut boundary, neutral structure, slices, and coverage without treating the region's shape as coordinate- or condition-invariant.
Color space	Color ScienceColor Representation	A defined coordinate system or organized inventory for representing colors, together with the encoding, reference conditions and transformations needed for reproducible interpretation.
Comminution	Particle ProcessingGeological Faulting	Mechanical breakup of solid material into smaller fragments or particles, whether deliberately operated or naturally occurring.
Compressed fluid	ThermodynamicsFluid Phase States	A liquid-state fluid at a pressure above its saturation pressure for the given temperature, equivalently below saturation temperature at the given pressure, and therefore not about to vaporize under that condition.
Condensation	ThermodynamicsPhase Transitions	Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization.
Confined Liquid	Condensed Matter Physics	A liquid restricted at micro- or nanoscopic dimensions so that geometry and interface interactions materially alter its structure, dynamics, transport, or phase behavior from the bulk state.
Conformal rotation vector	Attitude KinematicsThree Dimensional Rotation Parameterizations	Represent a three-dimensional orientation by stereographically projecting its unit quaternion to a three-component modified Rodrigues vector, with an explicit shadow-set switch around the chart singularity.
Conical refraction	OpticsCrystal Optics	Conical refraction is an optical phenomenon in which a ray of light, passing through a biaxial crystal along certain directions, is refracted into a hollow cone of light.
Continuous Slowing Down Approximation Range	Charged Particle Transport	CSDA range integrates reciprocal total stopping power to estimate a charged particle's path length while treating energy loss as continuous rather than fluctuating.
Contour Advection	Fluid DynamicsTracer Transport	A Lagrangian method that follows a materially conserved tracer contour through a fluid velocity field by advancing the contour's geometry along fluid trajectories.
Convective Overshoot	Fluid Dynamics	Convectively driven fluid motion that extends past a local stability boundary into an adjacent stably stratified region.
Cooling	ThermodynamicsHeat Transfer	Net reduction of a defined system's thermal energy, temperature, or lower-energy phase state through heat transfer or work-driven removal.
Cophonicity	Condensed Matter Physics	A vibrational-overlap metric measuring how strongly two atomic species contribute together within a selected frequency range.
Coriolis Force	Classical MechanicsRotating Frame Dynamics	In a rotating reference frame, account for the apparent transverse acceleration of relative motion with the exact velocity–rotation coupling -2 Omega cross v_rel.
Coulomb Operator	Quantum ChemistryHartree Fock Electronic Structure Theory	The Hartree–Fock one-electron operator that averages the pairwise Coulomb kernel over an occupied-orbital density and multiplies a test orbital by the resulting local repulsive potential.
Coulomb's law	ElectromagnetismElectrostatics	Relate the electrostatic force between ideal point charges to the product of their charges and the inverse square of their separation, directed along the line joining them and modified by the medium.
Counterfactual quantum computation	Quantum Information	A quantum protocol that infers a computational outcome from an interference branch in which the outcome-producing device did not run.
Coupled mode theory	Wave Physics and Photonics	A reduced perturbative framework that represents interacting waves or resonators by slowly varying modal amplitudes linked through coupling coefficients.
Crackling noise	Nonequilibrium Statistical Mechanics	A scale-spanning response regime in which a slowly driven system releases change through discrete avalanche-like events with characteristic size statistics.
Critical angle (optics)	OpticsGeometric Optics	The high-to-low-index incidence threshold at which refracted propagation becomes tangential and then evanescent, producing total internal reflection.
Critical dimension	Critical Phenomena	A spatial dimension at which the existence or universality behavior of a phase transition changes, including lower and upper critical dimensions.
Critical point (thermodynamics)	ThermodynamicsPhase Transitions	A thermodynamic state where a phase-coexistence curve terminates, the two phases lose their macroscopic distinction, and critical fluctuations emerge.
Cross-entropy benchmarking	Quantum BenchmarkingQuantum Computing	Cross-entropy benchmarking (XEB) is a statistical measure used to evaluate the performance in random circuit sampling experiments.
Crow instability	Fluid DynamicsVortex Dynamics	A growing long-wave bending mode of a coupled antiparallel vortex pair, assessed relative to the pair's base motion.
Crystal Ball function	Statistical Modeling in High Energy Physics	A probability-density shape joining a Gaussian core continuously and differentiably to a one-sided power-law tail.
Crystal momentum	Solid State Physics	In solid-state physics, crystal momentum or quasimomentum is a momentum-like vector associated with electrons in a crystal lattice.
CSS Code	Quantum Information ScienceQuantum Error Correction	A quantum stabilizer-code construction that imports two suitably nested classical binary linear codes so bit-flip and phase-flip syndromes can be checked with separate X-type and Z-type stabilizers.
Current algebra	Mathematical PhysicsInfinite Dimensional Lie Algebras	An infinite-dimensional Lie algebra of Lie-algebra-valued functions on a manifold, arising physically from equal-time commutators of conserved current densities.
Curved spacetime	General Relativity and Lorentzian Geometry	A spacetime modeled by a nonflat Lorentzian metric whose curvature encodes gravitational geometry and shapes free-fall trajectories, clocks, light propagation, and geodesic deviation.
Cymatics	AcousticsModal Vibration	Cymatics (from ) is a subset of modal vibrational phenomena.
Davydov Soliton	BiophysicsNonlinear Dynamics	A theoretical self-trapped amide-I excitation coupled to deformation of an alpha-helical peptide lattice, forming a localized traveling quasiparticle solution of the Davydov model.
De Donder–Weyl theory	Mathematical Physics	A covariant Hamiltonian formulation of classical field theory treating space and time coordinates symmetrically through polymomenta.
Debye Model	Solid State PhysicsLattice Vibrations and Heat Capacity	A continuum phonon model of a crystalline solid that uses linearly dispersing acoustic modes and a mode-count-preserving cutoff to derive the lattice heat capacity, including the low-temperature T³ law and high-temperature Dulong–Petit limit.
Decay energy	Nuclear Physics	The decay energy is the energy change of a nucleus having undergone a radioactive decay.
Deferred Measurement Principle	Quantum Information and Computation	A quantum-circuit equivalence that replaces intermediate measurement-dependent gates with coherent control so relevant outcomes and outputs are preserved when readout is postponed.
Defocus Aberration	Optical Imaging	Defocus aberration degrades an image when the observation plane does not meet the system's best-focus condition for a specified object point.
Deformation quantization	Mathematical PhysicsQuantization	A quantization method that replaces the commutative product of classical observables with a formal parameter-dependent noncommutative star product whose zeroth-order limit is classical multiplication and first-order commutator recovers the Poisson bracket.
Demon algorithm	Statistical Mechanics Computation	A microcanonical Monte Carlo method that augments a simulated system with a small energy reservoir whose exchanges enforce fixed total energy and reveal temperature.
Density Current	Physical SciencesFluid Dynamics	A fluid or fluidized mixture propagates through or beneath an ambient medium because a density contrast converts gravitational potential into a directed intrusion with a moving front and a stratified interface.
Density matrix	Quantum MechanicsQuantum Statistical Mechanics	A positive trace-one quantum state operator that determines all measurement probabilities and encompasses pure, statistical-mixture, and reduced entangled-subsystem states.
Dephasing rate SP formula	Mesoscopic PhysicsQuantum Decoherence	Using perturbation theory, one recovers at finite temperatures at the long time limit F(t)=\Gamma_{\varphi}t , where the decay constant is given by the dephasing rate formula with non symmetrized spectral functions as expected.
Deslandres table	Molecular SpectroscopyVibronic Spectroscopy	In electronic spectroscopy, constructing a Deslandres table is a useful method to assign vibronic transitions.
Diagrammatic Monte Carlo	Computational Physics	A stochastic numerical method sampling and summing selected Feynman-diagram expansions for interacting many-body systems.
Diamagnetism	Condensed Matter Physics	Diamagnetism is a recurring condensed-matter physics, magnetism identity in which an applied magnetic field induces an opposing magnetic response and therefore weak repulsion.
Diamond norm	Quantum InformationNorms on Quantum Operations	The completely bounded trace norm of a linear map on matrices, maximizing output trace norm after arbitrary ancillary extension and operationally measuring one-use distinguishability of quantum channels.
Dielectrophoresis	ElectrokineticsDielectric Particles	A nonuniform electric field exerts an induced-polarization force on a particle relative to its medium, drawing it toward or away from stronger-field regions under specified conditions.
Diffraction	Wave Optics	Wave-field spreading and interference around an aperture, edge, or obstacle, producing patterns that straight-ray propagation cannot explain.
Diffusing-wave spectroscopy	Soft Matter PhysicsOptical Spectroscopy	A multiple-scattering optical correlation method that uses coherent speckle decorrelation and photon-path statistics to infer microscopic dynamics, and conditionally rheology, in turbid soft materials.
Diffusion-limited aggregation	Statistical Physics and Fractal Growth	A stochastic growth process in which randomly diffusing particles irreversibly attach upon first contact with a cluster, producing branched scale-dependent aggregates.
Diffusive–Thermal Instability	Combustion TheoryReactive Flows	Growth of a flame disturbance when differential reactant and heat diffusion reinforces local differences in burning rather than damping them.
Diffuson	Diagrammatic Perturbation Theory	A diffuson is the disorder-averaged particle-hole ladder propagator whose low-energy pole describes diffusion in a disordered electronic conductor.
Dilaton	Theoretical PhysicsString Theory	A hypothetical scalar field whose expectation value parametrizes a coupling or scale, notably the effective string coupling or the size of compact extra dimensions in theories of gravity and unification.
Dim Spot	GeophysicsSeismic Amplitude Interpretation	A localized reduction of seismic reflection amplitude that can result from a lower acoustic-impedance contrast, used as a conditional direct hydrocarbon indicator after tuning, lithology, processing, and fluid alternatives are tested.
Dimensional deconstruction	Theoretical Physics	A four-dimensional gauge-theory construction whose product groups and link fields reproduce a discretized extra dimension over an energy range.
Dimensionless Quantity	Dimensional AnalysisMetrology	A physical quantity whose dimensional expression reduces to one, making its numerical value invariant under coherent changes of base measurement units while retaining a declared quantity kind.
Dipole	ElectrodynamicsMagnetostatics	A localized electric or magnetic source whose oriented first moment supplies a dipolar field contribution, leading remotely when lower-order terms vanish.
Dirac Equation	Relativistic Quantum MechanicsSpinor Fields	The Lorentz-covariant first-order field equation for relativistic spin-one-half matter, with particle and antiparticle solution sectors.
Discrete ordinates method	Computational PhysicsAngular Discretization of Transport Equations	Approximate a transport equation by replacing its continuous angular variable and scattering integral with a weighted finite set of propagation directions, then solve the coupled direction-specific spatial equations.
Dissipation factor	Electromagnetism and OscillationsLoss and Quality Measurement	Express oscillatory loss as the ratio of dissipative to reactive response, equivalently the reciprocal of quality factor under a declared convention.
Dissipative Structure	Nonequilibrium Thermodynamics	A reproducible organized regime maintained in an open system far from thermodynamic equilibrium by ongoing flows of energy or matter and irreversible dissipation.
Double electron capture	Nuclear Decay Physics	A rare nuclear-decay mode in which two bound electrons are captured by two protons, converting them into neutrons while atomic number falls by two and mass number is unchanged.
Drell–Yan Process	Particle PhysicsQuantum Chromodynamics	The Drell–Yan process is studied both in fixed-target and collider experiments.
Dropwise Condensation	Condensation Heat TransferSurface Wetting	Vapor condenses on a surface as separate droplets rather than a continuous liquid film; droplet clearing sustains the mode.
Dynamic Aperture	Circular AcceleratorsNonlinear Beam Dynamics	The declared connected region of initial particle conditions that survive a specified number of turns in a circular accelerator.
Dynamic light scattering	Light ScatteringOptics	Dynamic light scattering (DLS) is a technique in physics that can be used to determine the size distribution profile of small particles in suspension or polymers in solution.
Dynamic scaling	Statistical Physics	A self-similarity relation in which an evolving observable collapses across time when amplitude and spatial variables are rescaled by characteristic exponents.
Dynamical Decoupling	Quantum InformationCoherent Control	Open-loop coherent control that sequences quantum operations to suppress unwanted system–environment coupling while retaining useful evolution.
Dynamical mean-field theory	Condensed Matter Theory	A nonperturbative many-body method that maps a correlated lattice model to a self-consistent quantum impurity problem with a frequency-dependent bath.
D’Alembert Operator	Relativistic Field Theory	The Lorentzian metric contraction of second derivatives—the relativistic wave operator whose flat-spacetime form combines a time second derivative with the oppositely signed spatial Laplacian.
Earnshaw paradox	Wave PhysicsAcousticsFluid Dynamics	In fluid dynamics, the Earnshaw paradox is a physical paradox related to considering sound waves in an ideal inviscid fluid.
Effective Action	Quantum Field Theory	A quantum-corrected action functional whose stationary condition gives equations for field expectation values and whose derivatives generate one-particle-irreducible correlation functions.
Effective Field Theory	Quantum Field Theory	A field-theoretic description organized for a specified energy range by its active degrees of freedom, symmetries, operator expansion, power counting, matching conditions, and controlled truncation error.
Effective Mass (Solid-State Physics)	Condensed Matter Physics	Replace a carrier's local crystal-band response with a free-particle-like scalar or tensor mass derived for a stated observable, wavevector region, and approximation regime rather than treating one number as an intrinsic particle constant.
Effective one-body formalism	Gravitational PhysicsRelativistic Two Body Dynamics and Waveforms	Map relativistic compact-binary dynamics onto a deformed test-particle problem, resum perturbative information into effective potentials, and attach radiation reaction and merger–ringdown descriptions to model the full coalescence.
Ehrenfest–Tolman effect	Relativistic Thermodynamics	The general-relativistic equilibrium condition that locally measured temperature varies with gravitational redshift so temperature times the norm of the stationary timelike Killing field is constant.
Eight-vertex model	Statistical MechanicsExactly Solvable Models	In statistical mechanics, the eight-vertex model is a generalization of the ice-type (six-vertex) models.
Eightfold way (physics)	Particle Physics	An SU(3)-flavor symmetry classification organizing hadrons into multiplets by isospin and hypercharge.
Einselection	Quantum FoundationsDecoherence	Environment-induced superselection, the decoherence process by which system–environment interaction preferentially preserves robust pointer states and suppresses observable interference between them.
Einstein Group	General RelativitySpacetime Symmetry Groups	Note that the Einstein group approaches—but never reaches—the Poincare group as the flat spacetime (special relativity limit) is approached.
Einstein solid	Statistical MechanicsSolid State Physics	The Einstein solid is a model of a crystalline solid that contains a large number of independent three-dimensional quantum harmonic oscillators of the same frequency.
Einstein–Brillouin–Keller method	Semiclassical Physics	A semiclassical quantization method assigning action-integral conditions, including Maslov phase corrections, to invariant tori of integrable classical systems.
Elasticity tensor	Linear Elasticity	A fourth-rank stiffness tensor mapping small elastic strain to stress, with material symmetry constraining its coefficients.
Electric Susceptibility Tensor	Crystal OpticsElectromagnetism in Media	The electric susceptibility tensor is the rank-two material response tensor that relates an anisotropic dielectric's induced polarization to an applied electric field in the linear regime.
Electromotive Force	ElectrodynamicsCircuit Theory	The signed electrical driving action per unit charge supplied by a source along a specified path or circuit contour.
Electronic anticoincidence	Radiation Detection Instrumentation	A detector-background suppression method that rejects a primary-channel event when a veto detector produces a time-coincident pulse identifying a likely unwanted particle or interaction.
Electron–Ion Recombination in Plasma	Plasma PhysicsAtomic Collisions	A free electron binds a positive ion in a plasma through an allowed energy-transfer pathway, lowering the ion's charge without necessarily neutralizing the plasma.
Electron–Nuclear Double Resonance	Electron Paramagnetic ResonanceSpin Spectroscopy	Detect nuclear-spin transitions through their effect on a coupled electron-resonance signal to resolve local hyperfine interactions.
Electrostriction	Dielectric Materials	A quadratic electromechanical coupling in which dielectric polarization produces strain that is even under polarization reversal, with tensor coefficients linking strain to polarization products.
Elliott formula	Semiconductor OpticsCondensed Matter Physics	A semiconductor-optics expression that decomposes interband absorption or emission near a band edge into discrete exciton resonances and a Coulomb-modified electron–hole continuum, with broadening parameters as needed.
End correction	AcousticsPipe Resonance	In acoustics, end correction is a short distance added to the length of a resonance pipe, in order to calculate the precise resonant frequency of the pipe.
Energy current	—	A flow representation for the rate and direction of energy transfer through space or across a boundary.
Energy Level Splitting	Quantum PhysicsSpectral Structure and Degenerate Perturbation	Resolve one degenerate quantum energy into two or more distinct eigenvalues by changing the Hamiltonian so that its action within the degenerate subspace is not proportional to the identity.
Energy operator	Quantum PhysicsQuantum Dynamics	The quantum time-derivative operator iℏ∂/∂t acting on a wavefunction, equal to Hamiltonian action on Schrödinger solutions.
Energy quality	Thermodynamics and Ecological EnergeticsWork Potential and Energy Hierarchies	The relative capacity of an energy form, at stated environmental conditions, to perform useful work or drive valued transformations rather than being unavailable and dissipated, commonly expressed through exergy or domain-specific quality hierarchies.
Energy Transfer	—	Energy transfer is a physical process in which a defined amount or rate of energy passes across a declared system boundary or between subsystems, fields, matter, or degrees of freedom through work, heat, radiation, mass flow, or another specified interaction while energy accounting and transformation are kept explicit.
Energy transformation	Physics and Energy Systems	The conversion of energy among kinetic, potential, thermal, chemical, electrical, radiant, nuclear, and other accounting forms while total energy is conserved within a declared system boundary.
Entanglement Distillation	Quantum Information TheoryQuantum Communication	An LOCC resource-conversion protocol that consumes many imperfect shared entangled states to produce fewer states with higher fidelity to a maximally entangled target.
Entanglement Swapping	Quantum Information	Uses a joint measurement on the inner members of two entangled links to establish conditional entanglement between their outer members.
Entanglement witness	Quantum Information TheoryEntanglement Theory	In quantum information theory, an entanglement witness is a functional which distinguishes a specific entangled state from separable ones.
Entropic gravity	Foundations of Physics	The hypothesis that gravitational dynamics emerge as an entropic or information-theoretic force rather than a fundamental interaction.
Equations for a falling body	Classical MechanicsKinematics	Equations for a falling body is a mathematical description of a body in free fall.
Equations of Motion	MechanicsDynamics	Physical evolution laws that relate a system's time-indexed state variables and their derivatives so admissible initial data determine its motion.
Equipartition theorem	Classical Statistical Mechanics	In classical thermal equilibrium, each eligible independent quadratic energy term has mean energy k_B T/2.
Escape-Cone Constraint	Geometric OpticsOptical Interfaces	The angular window through which internal light can propagate out of a higher-index medium into a lower-index medium at an optical interface.
Etherington's Reciprocity Theorem	Relativistic Cosmology	A metric-null-geodesic light-bundle reciprocity between source and observer area distances that yields luminosity–angular-distance duality when photons are conserved.
Euler angles	Rigid Body Kinematics	Three ordered rotation angles that parameterize a three-dimensional orientation relative to a reference frame under a declared axis sequence and active or passive convention.
Evaporation	ThermodynamicsPhase Change	Surface vaporization in which molecules escape from a liquid into the gas phase, with net rate governed by temperature, vapor pressure, ambient concentration, flow and available surface.
Ewald summation	Computational Physics	A convergent decomposition of periodic long-range interaction sums into rapidly decaying real-space and reciprocal-space contributions.
Exchange operator	Identical Particle Permutation Symmetry	Act on a many-particle quantum state by permuting two identical-particle labels, yielding a unitary involution whose symmetric and antisymmetric eigenspaces encode bosonic and fermionic exchange behavior.
Exothermic process	Thermodynamics	A thermodynamic process that transfers energy from the system to its surroundings as heat under a declared boundary and sign convention.
External Photoelectric Emission	Solid Surface Photoemission	Photon excitation in a solid can release electrons across its surface into vacuum.
Falling cat problem	Geometric Mechanics and Deformable Body Dynamics	The mechanics problem of how a deformable body can reorient while total angular momentum remains zero by cycling its internal shape through noncommuting configurations.
Faraday paradox	Electrodynamics	A class of electromagnetic-induction setups whose observed electromotive force appears inconsistent with a naive magnetic-flux-change calculation.
Faraday Wave	Interfacial Wave Dynamics	A standing wave at a fluid surface or liquid interface sustained by periodic vertical forcing through a parametric instability branch.
Fermi Acceleration	Cosmic Ray PhysicsDynamical Systems	A particle gains kinetic energy through repeated encounters with moving scattering structures or boundaries when the encounter dynamics let gains accumulate.
Fermi gas	Quantum Statistical Mechanics	An ideal quantum-statistical model of many noninteracting fermions occupying available one-particle states according to Fermi–Dirac statistics and the Pauli exclusion principle.
Fermi liquid	Condensed Matter PhysicsFermi Liquid Theory	An interacting-fermion state whose low-energy behavior is governed by long-lived Landau quasiparticles adiabatically connected to free fermions, with renormalized masses and interactions.
Fermion Parity Operator	Quantum Field Theory	The unitary Hermitian involution (-1)^F that acts as +1 on even-fermion-parity states and -1 on odd states, grading the Hilbert space and distinguishing even from odd operators.
Fermi–Pasta–Ulam–Tsingou problem	Nonlinear Dynamics	The nonlinear-lattice problem in which energy placed in a few modes nearly recurs instead of rapidly equipartitioning as naive ergodic expectations predicted.
Ferrimagnetism	Condensed Matter Magnetism	Cooperative magnetic order in which antiparallel sublattice moments are unequal, leaving a nonzero spontaneous net magnetization below an ordering temperature.
Fick's laws of diffusion	Transport PhenomenaContinuum Physics	Fick's laws relate diffusive flux to concentration gradient and concentration change to the divergence of that flux, yielding macroscopic transport equations from local conservation and a constitutive proportionality.
Fidelity of quantum states	Quantum InformationState Similarity	A bounded similarity measure for two quantum states based on the trace norm of their density-operator square roots, equaling transition probability for pure states under the squared convention.
Field (physics)	Classical Field Theory	A physical quantity assigned a scalar, vector, spinor, or tensor value at each relevant point of space and time, as in temperature, velocity, or electromagnetic fields.
Field Electron Emission	Surface PhysicsElectron Emission	Cold field electron emission lets electrons tunnel from a surface into vacuum through a barrier thinned by a strong local electric field.
Firewall Paradox	Quantum Gravity	Prove that three independently motivated black-hole postulates — unitarity, a smooth horizon, and monogamy of entanglement — become jointly inconsistent at the Page time, forcing every theory of quantum gravity to declare which one it weakens.
Flow Tracer	Fluid Dynamics	A distinguishable signal is transported with a fluid and interpreted against its source and behavior to infer connections, pathways, mixing, or timescales.
Flux Qubit	—	A Josephson-junction superconducting loop uses tunnel-coupled opposite persistent currents near half-flux bias as a controllable physical qubit.
Fock state	Quantum MechanicsSecond Quantization	A quantum number state with a definite occupation count in each field mode, forming an orthonormal occupation-number basis of bosonic or fermionic Fock space.
Folded optics	OpticsOptical Design	An optical architecture that bends a beam through mirrors, prisms, or related elements so its optical path is longer than the device's physical depth.
Force Chain	Granular PhysicsForce Networks	A connected path of grains carrying concentrated compressive contact forces through a jammed granular material.
Force spectrum microscopy	BiophysicsMicrorheology	An active-microrheology method combining intracellular tracer fluctuations with independently measured mechanical response to infer the frequency spectrum of aggregate nonthermal cytoplasmic forces.
Four-Dimensional Chern–Simons Theory	Quantum Field Theory	Four-dimensional Chern–Simons theory uses a mixed topological–holomorphic gauge field to construct integrable-model data.
Fourth, fifth, and sixth derivatives of position	Higher Order Kinematics	The fourth through sixth time derivatives of position: snap, crackle, and pop.
Fractionalization	Condensed Matter Physics	A correlated quantum system's effective excitations carry separated or fractionalized quantum-number content relative to ordinary constituent-like modes.
Fracton (Subdimensional Particle)	Quantum Many Body Theory	An isolated emergent excitation that cannot be moved alone by the finite-support local operations allowed in its many-body model.
Fraunhofer Diffraction Equation	Wave OpticsDiffraction Theory	A far-field wave approximation that maps the complex field across an aperture to an angular field by a Fourier integral.
Free Fall	Classical Mechanics and Gravitation	Classify and predict a body's motion by enforcing the gravity-only condition: after release, no dynamically significant support, drag, thrust, lift, tension, or other non-gravitational force acts, whether the body moves downward, upward, ballistically, or in orbit.
Free Surface	Free Surface FlowInterfacial Mechanics	A mobile fluid boundary whose shape and motion are determined with the flow by kinematic and stress conditions.
Free-air gravity anomaly	Geophysics	The difference between observed and theoretical gravity after correcting measurement elevation to a reference level without compensating for intervening terrain mass.
Frenkel–Kontorova model	Nonlinear and Condensed Matter Physics	A model of elastically coupled particles in a periodic substrate potential that captures competition between a preferred spacing and an imposed lattice.
Frequency-resolved optical gating	Ultrafast OpticsLaser Physics	Frequency-resolved optical gating (FROG) is a general method for measuring the spectral phase of ultrashort laser pulses, which range from subfemtosecond to about a nanosecond in length.
Fresnel diffraction	OpticsWave Optics	Near-field diffraction described by paraxial propagation with retained quadratic phase, producing patterns that depend on wavelength, aperture scale, and observation distance.
Fresnel Zone	Wave OpticsRadio Wave Propagation	An indexed band of intermediate points whose source-to-point-to-observer path exceeds the direct path by one half-wavelength interval.
Galilean Group	Classical MechanicsSpacetime Symmetry Groups	Without the translations in space and time the group is the homogeneous Galilean group.
Galileo's ship	Classical MechanicsHistory of PhysicsPrinciple of Relativity	Galileo's ship refers to two physics experiments, a thought experiment and an actual experiment, by Galileo Galilei, the 16th- and 17th-century physicist and astronomer.
Gamma matrices	Mathematical PhysicsClifford Algebra Representations	Matrices satisfying the Clifford anticommutation relations for a spacetime metric, used to represent spinors and linearize relativistic wave operators.
Gapped Hamiltonian	Quantum Many Body PhysicsSpectral Theory	A growing local quantum many-body Hamiltonian family whose lowest excitation stays a positive energy above its full ground-state space as system size grows.
Gauge theory	Theoretical PhysicsGauge Field Theories	A field theory whose action is invariant under spacetime-dependent transformations from a gauge group, requiring connection-like gauge fields that relate local choices and whose curvature represents physical field strength.
Gaussian Quantum Monte Carlo	Quantum Many Body Simulation	A quantum many-body simulation method that samples stochastic phase-space trajectories of a Gaussian operator representation of the density operator.
General relativity	Gravitation	Einstein's geometric theory of gravitation in which mass–energy curves spacetime and freely falling bodies follow its geodesics according to the field equations.
Generalized coordinates	Analytical Mechanics	A minimal set of independent parameters that uniquely specifies a mechanical system’s configuration subject to its constraints.
Generalized forces	Analytical MechanicsLagrangian Mechanics	In analytical mechanics (particularly Lagrangian mechanics), generalized forces are conjugate to generalized coordinates.
Generalized probabilistic theory	Quantum Foundations and Operational Physics	An operational framework representing physical preparations as states, measurements as effects, transformations as maps, and outcome probabilities through convex pairings, broad enough to compare classical, quantum, and hypothetical theories.
Geometric quantization	Mathematical Physics	A construction that seeks a quantum Hilbert space and observables from a classical symplectic phase space while preserving its geometric structures.
Geometrical Frustration	Condensed Matter PhysicsStatistical Mechanics	A geometric arrangement of interacting units makes their locally preferred relations impossible to satisfy simultaneously.
Geometrical Optics	Optics	Geometrical Optics is a recurring optics, optical engineering identity in which light is approximated as rays that propagate, reflect, refract, split, or absorb while diffraction and interference are excluded.
Gibbs free energy	Thermodynamics	A thermodynamic potential equal to enthalpy minus temperature times entropy that governs equilibrium and non-expansion work at fixed temperature and pressure.
Ginzburg Criterion	Statistical MechanicsCritical Phenomena	A fluctuation-versus-mean-field consistency test that identifies where a specified critical-point saddle approximation ceases to be reliable.
Glauber dynamics	Statistical MechanicsIsing ModelMonte Carlo Methods	In statistical physics, Glauber dynamics is a way to simulate the Ising model (a model of magnetism) on a computer .
Gnu Code	Quantum Error Correction	A gnu code encodes a logical qubit in parity-separated, binomially weighted Dicke states whose excitation weights are spaced by gap g across m=gnu symmetric physical qubits.
Gottesman–Kitaev–Preskill Code	Quantum Information ScienceBosonic Quantum Error Correction	Protect finite-dimensional quantum information in one or more bosonic modes by encoding logical states as a periodic phase-space lattice whose stabilizer syndromes reveal small displacement errors.
Gouy–Stodola Theorem	ThermodynamicsExergy Analysis	In thermodynamics and thermal physics, the Gouy–Stodola theorem is an important theorem for the quantification of irreversibilities in an open system, and aids in the exergy analysis of thermodynamic processes.
Gradient Network	Network ScienceComplex Networks	A directed network derived from an undirected substrate graph and a scalar potential, with each node pointing to the extremal-potential node in its closed neighborhood.
Grain Boundary	Materials Science	The high-energy interface where two misoriented crystal grains meet — a broken-periodicity region that becomes the preferential locus for diffusion, segregation, nucleation, and cracking, and whose quantity and quality set a material's strength and failure behavior.
Grand Unified Theory	Particle PhysicsGauge Unification	A particle-physics model that embeds the strong, weak and electromagnetic gauge interactions in one larger gauge symmetry at high energy.
Graph state	Quantum Information and Computation	A multiqubit stabilizer state constructed from a graph by preparing one qubit per vertex in a superposition state and applying a controlled-phase entangling operation along every edge.
Grassmann number	Mathematical Physics	An element of an exterior algebra generated by anticommuting variables, with odd generators squaring to zero.
Gravitational memory effect	General RelativityGravitational Waves	Gravitational memory effects, also known as gravitational-wave memory effects, are predicted persistent changes in the relative position of pairs of masses in space due to the passing of a gravitational wave (GW).
Greenberger–Horne–Zeilinger state	Quantum Information and Foundations	A multipartite entangled state formed by a coherent superposition of all subsystems in one basis state and all in its complementary basis state.
Group Velocity	Wave PhysicsDispersion and Wave Packets	Read the leading propagation velocity of a narrow-band wave packet's envelope from the local slope of its dispersion relation, while treating bandwidth, curvature, loss, and causality as explicit limits on that interpretation.
Group-Velocity Dispersion	OpticsWave Propagation	Measure how a selected wave mode's group delay per unit length changes with frequency, governing the leading dispersive phase of finite-band pulses.
Hamiltonian Fluid Mechanics	Fluid DynamicsGeometric Mechanics	Represent nondissipative continuum motion by a Hamiltonian functional and a canonical or noncanonical Poisson structure whose bracket generates the fluid equations and exposes invariants, symmetries, and stability constraints.
Hamiltonian Mechanics	Classical Mechanics	A reformulation of classical mechanics that represents a system by conjugate coordinates and momenta in phase space and generates its entire dynamics from a single scalar Hamiltonian through symplectic flow — making conservation, symmetry, and solvability systematic calculations.
Hartmann Number	Magnetohydrodynamics	A dimensionless magnetohydrodynamic number combining field strength, fluid conductivity and viscosity, and a declared length to compare magnetic and viscous flow scales.
Hartree–Fock method	Quantum Many Body Theory	A self-consistent mean-field approximation representing a many-fermion stationary state by one Slater determinant.
Hata Propagation Model	Radio PropagationLand Mobile Path Loss	A closed-form empirical model that estimates median land-mobile path loss from frequency, distance, antenna heights, and environment-specific corrections inside a declared validity envelope.
Heat Engine	Thermodynamics	A cyclic system that absorbs heat from a hot source, converts part into work through a working substance, and rejects the rest to a colder sink under second-law limits.
Heavy-Fermion Material	Condensed Matter PhysicsStrongly Correlated Electrons	In materials science, heavy fermion materials are a specific type of intermetallic compound, containing elements with 4f or 5f electrons in unfilled electron bands.
Helicity (fluid mechanics)	Vortex DynamicsFluid DynamicsTopological Fluid Mechanics	A velocity–vorticity integral tracking handed linkage in a fluid flow under stated conditions.
Herschel–Bulkley fluid	RheologyNon Newtonian Fluid Mechanics	The Herschel–Bulkley fluid is a generalized model of a non-Newtonian fluid, in which the strain experienced by the fluid is related to the stress in a complicated, non-linear way.
Hodograph	Applied Mathematics and MechanicsVector Diagrams and Kinematics	Represent a changing vector by fixing its tail at one origin and tracing the locus of its head, so geometry in vector space exposes magnitude, direction, and derivative relations such as velocity and shear.
Holstein–Herring method	Quantum ChemistryAsymptotic Exchange Energy	Extract the exponentially small exchange splitting between asymptotically degenerate molecular states from probability-current flux through a median surface, avoiding cancellation of nearly equal total energies.
Homes's law	SuperconductivityCondensed Matter Physics	In superconductivity, Homes's law is an empirical relation that states that a superconductor's.
Horndeski Theory	Gravitational PhysicsScalar Tensor Gravity	The four-dimensional single-scalar metric action family whose Euler-Lagrange field equations are constrained to remain second order.
Hubbard model	Condensed Matter PhysicsStrongly Correlated Electrons	The Hubbard model is a simplified model used to describe the transition between conducting and insulating systems.
Hubbard–Stratonovich transformation	Statistical Field Theory and Many Body Physics	An exact Gaussian integral identity that replaces a quadratic interaction with a linear coupling to an auxiliary field, converting interacting-particle expressions into field-integral form.
Huygens–Fresnel principle	Wave Optics	A wave-propagation principle treating every point on a wavefront as a source of secondary wavelets whose phase-sensitive superposition forms the later field.
Hydrodynamic Entrainment	Fluid DynamicsTurbulence	Net incorporation of surrounding fluid across the boundary of a turbulent jet, plume, or current.
Hydrogen-like Atom	Atomic Physics	A hydrogen-like atom or ion has one nucleus and exactly one bound electron, with nuclear Coulomb attraction governing its leading structure.
Hydrostatic equilibrium	Fluid MechanicsFluid Statics	In fluid mechanics, hydrostatic equilibrium, also called hydrostatic balance and hydrostasy, is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force.
Ice-Type Model	Statistical Mechanics	Model a four-coordinated lattice with arrow variables constrained to two-in/two-out at every vertex, then weight the six allowed local configurations to derive global statistical behavior.
Ideal Scalar Transmission Conditions	Heat TransferElectrostatics	Ideal scalar transmission conditions join two field regions across a perfect, source-free interface by matching a specified scalar and its consistently oriented constitutive normal flux.
Imaging Method	Imaging ScienceOptics	An imaging method is a repeatable measurement-and-reconstruction procedure that couples a physical or computational contrast mechanism, illumination or excitation, sensing geometry, sampling, calibration, and reconstruction or rendering rule to produce a spatially organized representation of a scene, specimen, material, or process.
Incompatibility of quantum measurements	Quantum InformationMeasurement Theory	The absence of a single joint quantum measurement whose marginals reproduce a given set of observables, generalizing noncommutativity to general measurements.
Indistinguishable Particles	Quantum Many Body Physics	Same-kind quantum particles whose individual-label exchange does not distinguish physical configurations, constraining many-particle states and counting.
Inertance	Acoustics and Fluid DynamicsLumped Fluid Networks	Quantify the pressure difference required to accelerate volume flow in a fluid element, functioning as the inertial coefficient in lumped acoustic and fluid-network models.
Inertial Frame of Reference	Classical MechanicsSpecial Relativity	Choose a frame in which a force-free body maintains constant straight-line velocity.
Information Causality	Quantum Information ScienceQuantum FoundationsPhysical Principles for Correlations	A proposed principle limiting Bob's information gain about Alice's unknown data to the n classical bits she sends, even with pre-shared nonsignalling correlations.
Inglis–Teller Equation	Plasma PhysicsStark Broadening	An approximate plasma-spectroscopy relation that infers charged-particle density from the last resolvable high-series atomic level before Stark-broadened lines merge.
Inharmonicity	Musical AcousticsAcoustics	In music, inharmonicity is the degree to which the frequencies of overtones (also known as partials or partial tones) depart from whole multiples of the fundamental frequency (harmonic series).
Interaction Picture	Quantum Mechanics	Re-express quantum dynamics relative to a chosen reference evolution so operators carry that evolution while states evolve under the transformed interaction.
Internal Measurement	Quantum MeasurementEndophysics	In quantum mechanics, internal measurement refers to the measurement of a quantum system by an observer (referred to as an internal observer or endo-observer).
Inverse problem for Lagrangian mechanics	Mathematical PhysicsCalculus of Variations	The problem of determining whether a given system of differential equations is equivalent to Euler–Lagrange equations for some Lagrangian and, if so, constructing one.
Inversion transformation	Mathematical Physics	A conformal coordinate transformation that maps a nonzero point to a reciprocal radial position and, with translations and rotations, extends Poincaré symmetry toward the conformal group.
Inverted Pendulum	Classical MechanicsDynamical Systems	A pendulum with its mass above its support, whose unforced upright equilibrium is locally unstable but can be stabilized by variant-specific actuation or drive.
Isentropic Nozzle Flow	Fluid MechanicsCompressible Flow and Gas Dynamics	Model compressible flow through a changing-area nozzle by coupling steady one-dimensional mass and energy conservation with reversible adiabatic state relations, exposing subsonic and supersonic branches, sonic choking, and the exact boundaries at which shocks, heat, or friction invalidate the model.
Isodiapher	Nuclear Physics	Likewise, nuclides with the same neutron excess (N − Z) are called isodiaphers.
Isolated System	ThermodynamicsSystem Classification and Boundary Conditions	A physical or thermodynamic system whose modeled boundary permits neither matter nor energy transfer with its surroundings over the interval and resolution under study.
Isothermal Process	ThermodynamicsThermodynamic Processes and Paths	A thermodynamic process constrained to a constant system temperature while heat, work, composition, phase, pressure, or volume may change consistently with the first and second laws.
Isotropic coordinates	General Relativity	Coordinates for a spherically symmetric spacetime in which the spatial metric is conformal to a Euclidean radial metric and coordinate light cones appear isotropic.
Jacobi coordinates	Classical MechanicsMany Body Mechanics	An N-body coordinate set of N−1 mass-weighted relative vectors plus the overall center of mass, separating internal geometry from translation.
Jahn–Teller effect	Molecular Physics	The Jahn–Teller effect is the spontaneous symmetry-lowering distortion of a nonlinear molecular or solid-state configuration with an electronically degenerate ground state, removing degeneracy and lowering energy.
Janzen–Rayleigh Expansion	Fluid DynamicsCompressible Potential Flow	A low-Mach regular perturbation method for compressible potential flow that expands the velocity potential and derived fields in powers—often even powers—of a reference Mach number around the incompressible solution.
Jellium	Condensed Matter PhysicsQuantum Many Body Theory	A homogeneous interacting-electron model with a uniform compensating positive background, used to isolate many-body electronic behavior from lattice structure.
Johnsen–Rahbek effect	Electroadhesion	An enhanced electroadhesive attraction arising at a metal–semiconductor or metal–polymer interface when applied voltage produces interfacial charge and localized contact fields.
Jones calculus	Polarization Optics	A two-component complex-vector and matrix formalism for transforming fully polarized coherent light through linear optical elements.
Jones diagram	Photographic Optics	A multiaxis Cartesian chart whose paired directions represent related photographic variables so equivalent field of view and image-format choices can be compared graphically.
Josephson effect	Superconductivity	Quantum current and phase phenomena across a weak link between superconductors, including zero-voltage supercurrent and voltage-driven oscillation.
Jost function	Mathematical Scattering Theory	A scattering-theory Wronskian whose zeros and analytic structure encode bound states, resonances, and phase shifts of a radial wave equation.
Joule expansion	Classical and Statistical Thermodynamics	An irreversible free expansion in which a gas initially confined in an insulated rigid vessel expands into an evacuated region after a partition is removed.
Judd–Ofelt Theory	Atomic SpectroscopyOptical Materials	A rare-earth 4f spectroscopy model relating host-dependent intensity parameters and ion tensor factors to bounded electric-dipole transition strengths.
K-theory (physics)	String Theory and Topological Condensed Matter Physics	The use of topological K-theory to classify stable charges and phases in physical theories.
Kapitsa–Dirac effect	Atom OpticsQuantum Optics	Coherent diffraction of a matter wave by the spatially periodic light-shift potential of a standing electromagnetic wave, producing quantized momentum components under regime-dependent pulse and interaction conditions.
Kaye effect	Rheology	A non-Newtonian fluid phenomenon in which a descending stream forms a temporary leaping jet from a mound of the same shear-thinning liquid.
kBT Thermal Energy Scale	Statistical MechanicsThermal Physics	The product of Boltzmann's constant and absolute temperature, used as a particle-scale thermal energy and as the denominator in dimensionless comparisons such as E/(kBT).
KCBS pentagram	Quantum Foundations	A five-cycle contextuality scenario whose compatible measurements violate a noncontextual hidden-variable inequality in a three-level quantum system.
Kerr–Schild perturbation	General Relativity	A null rank-one deformation of a Lorentzian background metric, formed from a scalar profile and a background-null direction.
Kinetic Scheme	Statistical Mechanics	A directed, rate-annotated network of coarse-grained states whose transition law generates time-dependent populations, fluxes, relaxation modes, and path probabilities under declared Markovian or memory-bearing assumptions.
Kinoshita–Lee–Nauenberg theorem	Quantum Field Theory	A conditional cancellation result for infrared and mass singularities in sufficiently inclusive perturbative transition probabilities over degenerate states.
Kirkwood approximation	Statistical MechanicsLiquid State Theory	The Kirkwood approximation for a discrete probability density function P(x_{1},x_{2},\ldots ,x_{n}) is given by.
Kitaev chain	Condensed Matter PhysicsTopological Superconductivity	A spinless one-dimensional lattice Hamiltonian with hopping and p-wave pairing that models Majorana end modes in its gapped topological regime.
KLM protocol	Quantum Information	A universal linear-optical quantum-computing scheme using single photons, passive optics, photodetection, ancillas, teleportation and error correction.
Koopman–von Neumann Classical Mechanics	Mathematical Classical MechanicsOperatorial and Hilbert Space Formulations of Hamiltonian Dynamics	Represent a measure-preserving classical phase-space flow as unitary linear evolution on an L² Hilbert space while preserving classical predictions through a commuting algebra of physical observables and Liouville dynamics.
Korteweg Stress	Continuum Fluid MechanicsCapillarity	A gradient-dependent capillary contribution to fluid stress that distributes interfacial force across a finite density or composition transition under a specified constitutive model.
Kovacs Effect	Nonequilibrium Statistical Mechanics	A slow-relaxing system can leave a matched reference value and return nonmonotonically, revealing memory of its preparation history.
Kramers–Kronig relations	Mathematical PhysicsLinear Response	Hilbert-transform relations connecting real and imaginary parts of a causal linear response function through analyticity in the upper complex-frequency half-plane.
Kramers–Wannier Duality	Statistical MechanicsLattice Models	A lattice-model strong–weak coupling correspondence that matches one model's high-temperature graphs to a dual model's low-temperature defects, relating normalized partition functions subject to boundary-sector conditions.
Kreft’s Dichromaticity Index	Colorimetry	A paired CIELAB hue-shift measure comparing a transparent sample's maximal-chroma state with fourfold lighter/thinner and darker/thicker states to quantify dichromatism.
Kármán vortex street	Fluid DynamicsVortex Shedding	An alternating periodic array of counter-rotating vortices shed downstream of a bluff body within a characteristic range of flow conditions.
Ladder Operator	Quantum MechanicsOperator Methods	An operator that maps eigenstates of a grading operator to zero or to eigenstates whose eigenvalues differ by a fixed upward or downward step.
Lagrangian Mechanics	Classical Dynamics	Lagrangian mechanics derives motion from a scalar Lagrangian in generalized coordinates by stationary action, often simplifying constrained systems.
Laminar flow	Fluid DynamicsFlow Regimes	Fluid moves in an organized regime without sustained turbulent velocity fluctuations, while its paths, mixing, and transition depend on the setting.
Landau Derivative	Gas DynamicsThermodynamicsCompressible Flow	A dimensionless thermodynamic derivative that characterizes the curvature of an isentrope and the density dependence of sound speed, thereby distinguishing classical from nonclassical nonlinear-wave behavior in compressible fluids.
Landauer formula	Mesoscopic PhysicsQuantum Transport	A mesoscopic-transport relation expressing coherent electrical conductance as the conductance quantum multiplied by the sum of transmission probabilities of the conductor's available quantum channels.
Landau–Levich problem	Fluid Dynamics	A thin-film fluid problem determining the coating deposited on a plate withdrawn slowly from a liquid bath through the balance of viscous, capillary and gravitational effects.
Landau–Zener formula	Quantum MechanicsNonadiabatic Transitions	An analytic two-level quantum-transition probability for a constant-coupling avoided crossing whose diabatic energy separation varies linearly in time from the remote past to the remote future.
Langevin Dynamics	Brownian Motion	A stochastic physical-dynamics model that evolves resolved coordinates under deterministic force, dissipative drag, and random forcing, with noise and drag balanced for a thermal equilibrium target.
Laplace expansion (potential)	Potential TheoryMultipole Expansion	In physics, the Laplace expansion of potentials that are directly proportional to the inverse of the distance ( 1 / r ), such as Newton's gravitational potential or Coulomb's electrostatic potential, expresses them in terms of the spherical Legendre polynomials.
Lattice Boltzmann Methods	Computational Fluid DynamicsKinetic Theory	A family of mesoscopic computational-fluid-dynamics methods that evolve discrete particle-distribution functions on a lattice through collision and streaming, recovering macroscopic density and momentum fields under declared lattice, equilibrium, relaxation, forcing, and boundary assumptions.
Lattice Model (Physics)	Statistical MechanicsMathematical Physics	In mathematical physics, a lattice model is a mathematical model of a physical system that is defined on a lattice, as opposed to a continuum, such as the continuum of space or spacetime.
Law of the wall	Fluid MechanicsTurbulent Wall Bounded Flow	A near-wall scaling law relating mean turbulent-flow velocity logarithmically to dimensionless distance from a solid boundary.
Laws of thermodynamics	Thermodynamics	The four foundational thermodynamic laws relating equilibrium and temperature, conservation of energy, entropy and process direction, and the limiting behavior of entropy near absolute zero.
Lee–Yang theory	Statistical MechanicsPhase Transitions	In statistical mechanics, Lee–Yang theory, sometimes also known as Yang–Lee theory, is a scientific theory which seeks to describe phase transitions in large physical systems in the thermodynamic limit based on the properties of small, finite-size systems.
Lemaître–Tolman Metric	General RelativityRelativistic Cosmology	A spherically symmetric pressureless-dust spacetime metric whose areal radius evolves from radial mass and energy data, admitting expansion and collapse.
Leonard–Merritt mass estimator	AstrophysicsStellar Dynamics	A projected mass estimator for a spherical stellar system using stars' sky positions and two proper-motion velocity components, designed to be insensitive to velocity anisotropy under its assumptions.
Level Repulsion	Spectral TheoryCoupled Modes	Coupled modes that would meet as a parameter changes instead separate into hybridized eigenmodes, leaving an avoided-crossing gap set by their nonzero coupling.
Levinthal's Paradox	Protein BiophysicsProtein Folding	Contrast the astronomical time required for random exhaustive sampling of protein conformations with rapid biological folding, proving that folding dynamics are strongly biased and structured.
Levitation (physics)	Mechanics and Field Control	The stable suspension of an object against gravity without direct mechanical contact, produced by an upward force together with restoring forces that resist displacement.
Lieb–Liniger model	Quantum Many Body PhysicsExactly Solvable Models	An exactly solvable model of identical bosons in one spatial dimension interacting through pairwise delta-function contact forces.
Linear elasticity	Continuum MechanicsSolid Mechanics	Linear elasticity is a mathematical model of how solid objects deform and become internally stressed by prescribed loading conditions.
Linear energy transfer	Radiation Physics and Dosimetry	The mean energy an ionizing particle deposits locally per unit path length in matter under a specified restricted or unrestricted convention.
Linear motion	Classical MechanicsKinematics	One-dimensional motion along a fixed straight axis, fully described by a signed position x(t) and its time derivatives for velocity and acceleration.
Linear optical quantum computing	Quantum InformationQuantum Optics	Linear optical quantum computing or linear optics quantum computation (LOQC), also photonic quantum computing (PQC), is a paradigm of quantum computation, allowing (under certain conditions, described below) universal quantum computation.
Liouville Dynamical System	Classical MechanicsHamilton Jacobi Separability	A class of natural mechanical systems whose kinetic metric and potential share Liouville's additive coordinate form, separating the motion into one-coordinate quadratures with energy and separation constants.
Liouville Space	Quantum PhysicsDensity Operator Formalism	The Hilbert space of Hilbert–Schmidt operators on a quantum Hilbert space, in which density operators are vectors and quantum evolutions act as superoperators.
Little Higgs	Particle PhysicsBeyond Standard Model	A model family in which the Higgs is a pseudo-Goldstone boson of collectively broken approximate symmetries, with partner loops canceling leading mass corrections.
Local reference frame	Local Frames in Spacetime	Represent observations relative to a basis or coordinate frame attached to a bounded neighborhood, point, observer, or subsystem, with transformations explicit.
Logarithmic frequency ratio	AcousticsUnits of Measurement	The logarithmic frequency ratio (also known as frequency level) of two frequencies is the logarithm of their ratio, and may be expressed using the unit octave (symbol: oct) corresponding to the ratio 2 or the unit decade (symbol: dec) corresponding to the ratio 10.
Lorentz oscillator model	OpticsDielectric Response Models	A classical model of bound charges as damped driven harmonic oscillators, producing frequency-dependent dielectric response, dispersion and resonant absorption.
Magnetic Anisotropy Energy	Magnetism	The orientation-dependent contribution to a magnetic system's energy that creates easy and hard magnetization directions through crystal symmetry, sample shape, stress, interfaces, or related magnetic couplings.
Magnetic helicity	Magnetohydrodynamics and Plasma Physics	A volume integral of magnetic vector potential dotted with magnetic field that measures field-line linkage, twist, and writhe under stated boundary and gauge conditions.
Magnetic Quantum Number	Quantum Mechanics	Label the discrete eigenvalue of an angular-momentum component along a declared quantization axis, with projection values mħ ranging from -jħ to +jħ in unit steps.
Magnetic vector potential	Electromagnetism	In classical electromagnetism, magnetic vector potential (often denoted A) is the vector quantity defined so that its curl is equal to the magnetic field, B: \nabla \times \mathbf{A} = \mathbf{B} .
Magnetocapacitance	Condensed Matter PhysicsMagnetic Field Dependent Capacitance	Measure a magnetic-field-dependent change in capacitance while separating intrinsic magnetoelectric response from magnetoresistive, interfacial, frequency, and contact artifacts.
Malter Effect	Electron EmissionSurface Physics	A radiation-induced dielectric charging feedback in which positive surface charge creates a field that drives continuing electron emission.
Manley–Rowe relations	Nonlinear Wave Theory	Conservation relations that constrain energy or action exchange among frequency components participating in a lossless nonlinear wave interaction.
Marangoni effect	Fluid DynamicsInterfacial Phenomena	The Marangoni effect (also called the Gibbs–Marangoni effect) is the mass transfer along an interface between two phases due to a gradient of the surface tension.
Marangoni number	Fluid Dynamics	A dimensionless ratio comparing surface-tension-gradient-driven transport with diffusion of the quantity producing that gradient.
Mass–Energy Equivalence	Special Relativity	A relativistic system's invariant mass equals its center-of-momentum energy divided by c squared; mass changes track rest-energy changes across a declared boundary.
Material derivative	Continuum Mechanics and Fluid Dynamics	The rate of change of a field observed while following a material particle moving through a continuum, equal to local time change plus advective transport.
Matrix Product State	Quantum Many Body PhysicsTensor Network Representations	A one-dimensional quantum-state representation that factorizes a many-site coefficient tensor into an ordered chain of local tensors, with virtual-bond dimensions controlling exact Schmidt ranks, approximation capacity, and contraction cost.
Maxwell Model	RheologyContinuum Mechanics	A linear viscoelastic model coupling an elastic spring and viscous dashpot in series, with common stress and additive strain.
Maxwell's Demon	ThermodynamicsStatistical Mechanics	A thermodynamic thought experiment in which an information-using agent selectively gates fast and slow molecules between chambers, seemingly lowering entropy until measurement, memory, and erasure are included in the system.
Mean-field theory	Statistical Mechanics	In physics and probability theory, mean-field theory (MFT) or self-consistent field theory studies the behavior of high-dimensional random (stochastic) models by studying a simpler model that approximates the original by averaging over degrees of freedom (the number of values in the final calculation of a statistic that are free to vary).
Measurement Method	MetrologyMeasurement ProceduresApplied Metrology	A repeatable documented procedure that operationalizes a measurand through sampling, sensing or comparison, calibration, calculation, quality control, and result reporting.
Mechanical Advantage	Classical MechanicsMachine Design	Mechanical advantage compares useful output with input force or, in rotary transmission, torque; its ideal ratio follows reciprocal motion under passive lossless work balance.
Mechanical Constraint	Classical Mechanics	A kinematic condition restricting the admissible configurations or instantaneous motions of a physical mechanical system.
Mechanical Similarity	Classical Mechanics	A homogeneous potential lets a classical trajectory generate scaled solutions when space and time are dilated by matched powers.
Metamaterial	ElectromagneticsOptics	A metamaterial is an engineered material or composite whose deliberately organized inclusions, resonators, or unit cells produce an effective response—often electromagnetic, acoustic, elastic, or mechanical—governed substantially by architecture and scale rather than constituent chemistry alone.
Method of continued fractions	Quantum Scattering Theory	The method of continued fractions is a method developed specifically for solution of integral equations of quantum scattering theory like Lippmann–Schwinger equation or Faddeev equations.
Method of image charges	Electrostatics	An electrostatic solution method that replaces conductor boundaries with fictitious charges chosen to reproduce the required boundary conditions.
Metric System	MetrologySystems of Units	A coherent measurement-unit family built from defined base units, systematically derived units, and standardized decimal prefixes for powers-of-ten multiples and submultiples.
Metropolis Algorithm	Statistical PhysicsComputational PhysicsMarkov Chain Monte Carlo	In statistics and statistical physics, the Metropolis–Hastings algorithm is a Markov chain Monte Carlo (MCMC) method for obtaining a sequence of random samples from a probability distribution from which direct sampling is difficult.
Microrheology	Soft MatterRheology	A rheological measurement method that infers local viscoelastic response from the thermally driven or externally forced motion of microscopic tracer particles embedded in a material.
Minor losses in pipe flow	Fluid MechanicsInternal Pipe Flow and Hydraulic Networks	Model localized irreversible mechanical-energy losses caused by fittings, valves, entrances, exits, bends, and area changes as a dimensionless coefficient multiplying a declared velocity head, while separating them from length-distributed wall-friction loss.
Mirror nuclei	Nuclear Structure	A pair of isobars whose proton and neutron counts are exchanged, providing a symmetry comparison of nuclear structure.
Mirror symmetry (string theory)	Mathematical Physics	A duality pairing Calabi–Yau geometries whose associated string compactifications are physically equivalent while exchanging complex and symplectic geometric data.
Mixing Length Model	Fluid MechanicsTurbulence Modeling	A turbulence closure that models eddies as carrying mean-flow properties over a characteristic distance before mixing, converting mean gradients into turbulent fluxes.
Modified Pressure	Fluid DynamicsIncompressible Momentum Equations	Absorb a conservative body-force potential into fluid pressure so the momentum equation depends on one combined gradient, while retaining a recovery rule for true mechanical pressure and explicit limits at variable density and pressure-sensitive boundaries.
Modular Invariance	Conformal Field TheoryString Worldsheet Theory	Preservation of an appropriately assembled torus partition function or one-loop quantity under modular changes of its cycle basis.
Moffatt eddies	Fluid DynamicsStokes Flow	Moffatt eddies are sequences of eddies that develop in corners bounded by plane walls (or sometimes between a wall and a free surface) due to an arbitrary disturbance acting at asymptotically large distances from the corner.
Mohr's Circle	Continuum MechanicsPlane Stress Transformation	Encode a symmetric plane-stress state as a circle in normal–shear coordinates, so a physical plane rotation appears at double angle and principal stresses, maximum in-plane shear, and their orientations can be read from one invariant locus.
Moiré pattern	Wave Optics Imaging and Visual Design	A large-scale beat or interference pattern produced by superposing similar periodic structures with a small mismatch in spacing, orientation, curvature, or sampling.
Momentum	Classical Mechanics	Capture a system's quantity of motion as the conserved additive vector p = mv, so any interaction — however messy inside — collapses to an algebraic balance where the total before equals the total after up to external impulse.
Momentum-Transfer Cross Section	Scattering TheoryKinetic Transport	Summarize an elastic scattering angle distribution by weighting each event for its loss of incident-direction momentum.
Montonen–Olive Duality	Supersymmetric Quantum Gauge Theory	A strong–weak equivalence conjecture in supersymmetric gauge theory that exchanges electric gauge particles with magnetic monopoles, coupling with its reciprocal, and Noether with topological charge.
Mott–Schottky Equation	Semiconductor Physics	A depletion-layer capacitance relation for semiconductor junctions in which 1/C² varies linearly with applied potential, enabling estimates of dopant density and flat-band potential under model assumptions.
Multi-Configuration Time-Dependent Hartree	Quantum DynamicsWavepacket Propagation	A variational quantum-dynamics method that expands a multidimensional wavefunction in configurations built from time-dependent single-particle functions and evolves both coefficients and basis functions.
Multifractal system	TurbulenceStatistical Physics	A multifractal system is a generalization of a fractal system in which a single exponent (the fractal dimension) is not enough to describe its dynamics; instead, a continuous spectrum of exponents (the so-called singularity spectrum) is needed.
Muon Capture	Particle and Nuclear PhysicsWeak Capture Processes	Weak absorption of a negative muon by a proton, producing a neutron and muon neutrino, with optional nuclear or radiative products.
Muon spin spectroscopy	Condensed Matter PhysicsMuon Spectroscopy	Muon spin spectroscopy, also known as μSR, is an experimental technique based on the implantation of spin-polarized muons in matter and on the detection of the influence of the atomic, molecular or crystalline surroundings on their spin motion.
Mushroom Cloud	Explosion PhysicsBuoyant Cloud Rise	A transient stem-and-cap cloud formed when an impulsively heated, buoyant mass rises through a stratified atmosphere, entrains surrounding fluid and debris, rolls into a toroidal head, and spreads as its density approaches the environment.
M–Sigma Relation	AstrophysicsBlack Hole and Galaxy Demographics	An empirical, approximately power-law correlation between a galaxy's central supermassive-black-hole mass and the stellar velocity dispersion of its spheroidal component, used as a demographic scaling relation whose calibration and interpretation depend on sample, morphology, measurement, and selection.
N-body simulation	Computational Astrophysics	Numerically evolve many interacting particle representatives by repeatedly evaluating forces, advancing states, and controlling approximation and integration error.
Nahm Equations	Gauge TheoryMathematical Physics	Evolve a Lie-algebra-valued triple by cyclic commutator ODEs, modulo gauge and application-specific boundary data, while a Lax representation preserves spectral data.
Natural Units	MetrologyTheoretical Physics	In physics, natural unit systems are measurement systems for which selected physical constants have been set to 1 through nondimensionalization of physical units.
Near-Field Radiative Heat Transfer	Nanoscale Heat Transfer	Thermally driven electromagnetic exchange across subwavelength gaps, where evanescent channels supplement propagating radiation and can carry heat far above the corresponding far-field blackbody benchmark.
Nernst Effect	Condensed Matter PhysicsThermoelectric Transport	Measure the ordinary off-diagonal thermoelectric response in which a longitudinal temperature gradient and perpendicular applied magnetic field generate a transverse open-circuit electric field, while keeping the response geometry distinct from its several microscopic mechanisms.
Network Communication Efficiency	Network ScienceComplex Networks	A shortest-path-based network measure that assigns pairwise efficiency as reciprocal distance and summarizes global or local potential exchange under a declared graph convention.
Neutrinoless double beta decay	Nuclear PhysicsNeutrino Physics	A hypothetical nuclear transition emitting two electrons but no neutrinos, whose observation would violate total lepton number and demonstrate a Majorana component of neutrino mass; no confirmed detection currently exists.
Neutron Spectroscopy	Neutron Scattering	A family of spectroscopic measurements that determines neutron energy distributions or energy-and-momentum changes, using known source and detector response to infer atomic motion, magnetic excitations, nuclear processes, or source-plasma properties.
Newton's cannonball	Mechanics PedagogyCelestial MechanicsThought Experiments	In this experiment described near the start of his De mundi systemate, Newton visualizes a stone being projected from the top of a high mountain, and "that there is no air about the earth, or at least that it is endowed with little or no power of resisting".
Newton's Laws of Motion	Classical MechanicsDynamics	Three linked classical-dynamics laws that identify inertial motion, relate net force to momentum change, and pair mutual forces on interacting bodies.
Nilsson model	Nuclear Structure Physics	The Nilsson model treats a nucleus as a deformed shell potential to make collective rotational spectra tractable.
No-cloning theorem	Quantum InformationQuantum No Go Theorems	The quantum-information theorem that no physical operation can produce a perfect independent copy of every arbitrary unknown quantum state while retaining the original.
Non-Contact Atomic Force Microscopy	Surface ScienceAtomic Force Microscopy	Non-contact AFM scans an oscillating probe near a surface and maps force-induced changes in its motion without sustained tip–surface contact.
Non-Contact Force	Classical Mechanics	A non-contact force is a force which acts on an object without coming physically in contact with it.
Non-Linear Sigma Model	Quantum Field Theory	Represent fields as maps from spacetime or another base into a curved target manifold, with dynamics governed by the pullback of the target metric and any explicitly added potential or topological terms.
Non-local quantum computation	Quantum InformationDistributed Quantum Computation	A distributed quantum-computation model in which separated parties use pre-shared entanglement and a single simultaneous communication round to implement a joint operation on distributed inputs.
Noncommutative quantum field theory	Mathematical Physics	A quantum-field-theory framework defined on a spacetime whose coordinate algebra does not commute.
NOON State	Quantum OpticsQuantum MetrologyEntanglement	In quantum optics, a NOON state or N00N state is a quantum-mechanical many-body entangled state.
Nordtvedt Effect	Experimental GravitationOrbital Dynamics	A conditional orbital polarization caused by different free-fall responses of self-gravitating bodies with unequal binding-energy fractions in an external gravitational field.
Nuclear Clock	Time and Frequency MetrologyNuclear PhysicsQuantum Metrology	A proposed or developing clock whose reference oscillator is locked to a narrow nuclear isomer transition rather than an electronic atomic transition, aiming for exceptionally stable optical-frequency timekeeping.
Nuclear drip line	Nuclear PhysicsNuclear Structure	The nuclear drip line is the boundary beyond which atomic nuclei are unbound with respect to the emission of a proton or neutron.
Nuclear Fission	Nuclear PhysicsNuclear Reactions	A nuclear reaction in which a heavy atomic nucleus splits into lighter nuclei, releasing binding energy as fragment motion and radiation and often emitting neutrons.
Nuclear resonance fluorescence	Nuclear PhysicsGamma Spectroscopy	Nuclear resonance fluorescence (NRF) is a nuclear process in which a nucleus absorbs and emits high-energy photons called gamma rays.
Nuclear shell model	Nuclear PhysicsNuclear Structure	A quantum model of atomic nuclei in which protons and neutrons occupy quantized single-particle orbitals in an average potential, with Pauli filling and strong spin–orbit coupling explaining magic numbers, spins, parities, and shell-dependent stability.
Null Hypersurface	General RelativityLorentzian Geometry	A smooth spacetime hypersurface whose normal is lightlike and tangent to it, leaving a degenerate induced metric along null-geodesic generators.
Objective Stress Rate	Finite Deformation Mechanics	An objective stress rate corrects the rate of a spatial stress tensor for rigid rotation so a rate-form material law does not mistake a change of frame for deformation.
Observable	Quantum MechanicsMeasurement	In physics, an observable is a physical property or physical quantity that can be measured.
Okorokov effect	Atomic and Crystal Physics	Resonant coherent excitation of fast ions channeled through a crystal when their internal transition frequency matches the periodic lattice-field frequency in the ion frame.
Opacity	OpticsRadiative Transfer	The resistance of a material or medium to radiation transmission at specified frequencies, produced by absorption, scattering, or reflection and quantified in contexts such as radiative transfer by attenuation per unit mass.
Optical Coherence Tomography	OpticsOptical Imaging	An optical imaging method that turns reference-interferometric depth profiles of returned light into cross-sections of internal layered structure.
Optical heterodyne detection	OpticsCoherent Detection	A coherent optical detection method that mixes a signal with a frequency-offset local oscillator so phase and frequency modulation appear as an electronically measurable beat signal.
Optical neural network	Photonic ComputingOptics	An optical neural network is a physical implementation of an artificial neural network with optical components.
Optical resolution	OpticsImaging Systems	The smallest feature scale or separation an optical imaging system can distinguish under stated wavelength, aperture, contrast, sampling, and decision criteria.
Optical space	Geometrical Optics	A coordinate-bearing mathematical view of an optical system associated with a refractive region such as object, image or intermediate space.
Optical Transfer Function	Image FormationTransfer Function Analysis	Normalized complex spatial-frequency response of a specified locally shift-invariant optical intensity-imaging system, combining modulation and phase transfer.
Optical Vortex	OpticsSingular Optics	A topological phase defect of a coherent optical field where complex amplitude vanishes, phase is undefined, and phase accumulated around an enclosing loop is an integer multiple of 2π, giving the singularity a signed winding charge.
Optogenetic methods to record cellular activity	Cellular BiophysicsGenetically Encoded Optical Reporting	Use genetically encoded optical indicators whose light output changes with a cellular variable to record spatially resolved activity while preserving the distinction between sensing and optical actuation.
Orientifold	String TheoryString Compactification	A string-theory quotient that combines a spacetime symmetry with worldsheet-orientation reversal, producing unoriented sectors and characteristic fixed-plane and consistency conditions.
Oscillator linewidth	OscillatorsSpectral Metrology	The frequency spread of a selected physical oscillator output line, reported with a specified spectral estimator and width convention.
Overblowing	AcousticsMusical Instruments	Selecting a playable upper register of a wind instrument while retaining a lower-note fingering or sounding-length reference.
Pair Distribution Function	Statistical MechanicsCondensed Matter Physics	A normalized radial function comparing particle-pair occurrence at each separation with the occurrence expected from the system's mean number density.
Participation (philosophy)	Metaphysics	A metaphysical relation in which a particular or dependent being has a form, property or reality through an asymmetric dependence on what it participates in.
Particle decay	Particle Physics	Particle decay denotes spontaneous process of one unstable subatomic particle transforming into multiple other particles within particle physics.
Particle in a one-dimensional lattice	Solid State and Quantum Physics	The quantum model of a particle moving in a spatially periodic one-dimensional potential, whose stationary states have Bloch form and organize into energy bands separated by gaps.
Particle in a spherically symmetric potential	Quantum Mechanics	In quantum mechanics, a particle in a spherically symmetric potential is a system where a particle's potential energy depends only on its distance from a central point, not on the direction.
Particle statistics	Quantum Statistical Mechanics	The exchange-linked occupation rules and ideal-gas equilibrium laws for identical bosons and fermions, with classical counting recovered in the dilute limit.
Particle tracking velocimetry	Experimental Fluid MechanicsFlow Diagnostics	An image-based flow-measurement method that links individual tracer detections across known times to reconstruct measured trajectories and estimate velocity.
Particle-in-Cell Method	Computational Plasma PhysicsKinetic Simulation	A hybrid kinetic simulation loop that advances Lagrangian macroparticles, scatters their sources to a mesh, solves self-consistent fields there, and gathers forces back to the particles.
Partition Function	Equilibrium Statistical Mechanics	A normalization sum or integral over a statistical-mechanical ensemble's microstates, weighting each state by its ensemble factor and generating equilibrium probabilities and thermodynamic potentials.
Path Integral Formulation	Quantum MechanicsQuantum FormulationsQuantum Field Theory	A quantum formulation that obtains amplitudes from an action-weighted functional sum over possible histories.
Pauli Matrices	Quantum MechanicsMathematical Physics	In mathematical physics and mathematics, the Pauli matrices are a set of three 2 \times 2 complex matrices that are traceless, Hermitian, involutory and unitary.
Penrose diagram	General RelativityCausal Spacetime Diagrams	A conformal spacetime diagram that compresses infinity to a finite boundary while preserving causal light-cone structure.
Penrose–Hawking Singularity Theorems	General RelativityGlobal Lorentzian Geometry	A family of global Lorentzian-geometry theorems deriving causal geodesic incompleteness from focusing, energy or convergence, causality, and trapped-or-expanding initial conditions.
Peres–Horodecki criterion	Quantum InformationEntanglement Detection	Test bipartite quantum-state separability by partially transposing one subsystem and checking positivity, a necessary condition in all dimensions and a sufficient one only for 2×2 and 2×3 systems.
Periodic Table of Topological Insulators and Topological Superconductors	Topological Band TheoryFree Fermion Phases	A tenfold table mapping a gapped free-fermion system's symmetry class and spatial dimension to its possible stable topological classification group.
Phase space crystal	Condensed Matter Physics	A state exhibiting discrete translational or rotational order in phase space rather than ordinary real-space position.
Phase Synchronization	Nonlinear Dynamics	Lock a bounded integer combination of oscillator phases while allowing amplitudes and other state variables to remain different or irregular.
Phase Velocity	Wave PhysicsDispersion	The velocity at which a point of constant phase of a single-frequency wave component propagates, equal in one dimension to angular frequency divided by wavenumber and distinct from envelope, energy, or information velocity.
Phase-Space Measurement with Forward Modeling	Biomedical OpticsComputational Imaging	Phase space measurement with forward modeling is one approach to address the scattering issue in biomedical imaging.
Phason	Aperiodic CrystalsIncommensurate Order	An internal phase displacement degree or mode of aperiodic order, distinct from ordinary physical translation and amplitude change.
Philosophical realism	Metaphysics	The stance that entities of a specified kind exist or have facts independently of minds, conceptual schemes, language, or acts of knowing.
Phosphorescence	Delayed Photoluminescence	Emit delayed luminescence after excitation because population reaches a comparatively long-lived state whose radiative return is quantum-mechanically disfavored or otherwise kinetically inhibited.
Photoacoustic imaging	Biomedical ImagingAcoustics	A spatial imaging method that detects thermoelastic sound generated by time-varying electromagnetic absorption in a target.
Photomagnetism	Condensed Matter and Materials PhysicsMolecular Magnetism	A light-induced change in magnetic order or magnetization arising from photoactive electronic and spin conversion under specified material, wavelength, temperature, and history conditions.
Physical and Logical Qubits	Quantum InformationQuantum Error Correction	The quantum-computing layer boundary that realizes an algorithm-facing qubit as encoded states and operations in noisier physical degrees of freedom.
Physical optics	Optics	The treatment of optical propagation as waves so interference, diffraction and polarization are retained beyond geometric rays.
Physical Potential	—	A physical potential is a scalar, vector, or more general field introduced so that a physically observable force, field, energy relation, or dynamical effect can be derived from it by a specified differential or variational operation, subject to boundary conditions and possible gauge freedom.
Physical-System Model	Theoretical PhysicsPhysical Modeling	An idealized representation specifying physical entities or fields, state variables, governing relations, conditions, and an observation map so a physical system can be explained, simulated, or predicted.
Physicalism	Metaphysics	The metaphysical thesis that everything is physical or wholly determined by the physical.
Planck Units	Theoretical Physics	Build a natural unit system from powers of c, G, ħ, and k_B so those constants have numerical value one and physical quantities are expressed relative to corresponding Planck scales.
Poincaré group	Relativistic Physics	The Lie group of all isometries of Minkowski spacetime, combining Lorentz transformations with spacetime translations.
Point particle	Theoretical Physics	An idealized physical object with finite properties such as mass or charge but zero spatial extent at the modeling scale.
Polarimetry	Polarimetric Measurement	Measurement and analysis of a wave's polarization state or polarization-resolved response under declared acquisition conditions.
Polaritonics	Applied PhysicsTerahertz Polaritonics	Terahertz signal processing that generates and controls guided phonon-polaritons in polar media.
Polarization (waves)	Wave Physics	The geometric pattern traced by a transverse wave’s oscillating field or displacement in the plane perpendicular to propagation.
Pole Mass	Quantum Field TheoryParticle Physics	A particle-mass definition associated with the pole of its fully corrected propagator, contrasting with scale- and scheme-dependent running mass parameters and requiring qualification for unstable, confined, or infrared-sensitive states.
Polymer Scattering	Polymer PhysicsScattering Methods	A polymer-characterization framework that interprets wave-vector-dependent coherent scattering intensity as weighted intra- and inter-chain correlations to infer chain size, conformation, interactions, and mesoscale organization under explicit contrast and model assumptions.
Polynomial hyperelastic model	Continuum MechanicsHyperelastic Constitutive Models	A phenomenological finite-strain material model expressing strain-energy density as a polynomial in invariants of the isochoric deformation tensor plus an optional volumetric term.
Poole–Frenkel Effect	Solid State Electronics	Field-enhanced thermal emission from a charged bulk trap in a dielectric or semiconductor, where an applied electric field lowers the Coulombic escape barrier and increases trap-assisted conduction.
Positron Annihilation Spectroscopy	Materials PhysicsExperimental Condensed Matter	A materials-probing method that interprets positron-annihilation timing or photon energy/momentum signatures through stated references and models to characterize defects or free volume.
Potential Energy	Classical Mechanics	Energy associated with a system's position or configuration under conservative interactions, defined up to an additive reference and changed by the negative work of the force.
Power (Physics)	MechanicsElectrical Physics	The rate of work or physical energy transfer or conversion assigned to a specified process and time base.
Power-to-weight Ratio	Power and Mass Metrics	Power-to-weight ratio states a specified physical power output per unit of a declared mass, with operating basis and system boundary needed to interpret the resulting W/kg figure.
Poynting Vector	ElectromagnetismElectromagnetic Energy Conservation	The local electromagnetic energy-flux density whose surface-normal integral gives electromagnetic power crossing a boundary and whose divergence closes the field-energy balance.
Preisach model of hysteresis	Magnetic Hysteresis	A rate-independent hysteresis model that represents path-dependent output as a weighted superposition of two-threshold relay operators retaining state between threshold crossings.
Pressure	Fluid MechanicsContinuum Mechanics	Pressure is a signed normal-force-per-area intensity, isotropic in a fluid at rest, whose absolute or gauge reference and flow conditions must be stated before applying further laws.
Pressure–volume diagram	Thermodynamics	A plot of system pressure against volume along one or more processes, whose path shape represents state change and whose enclosed or signed area represents boundary work.
Primakoff Effect	Particle Physics	A coherent electromagnetic-field conversion between a photon and a neutral pseudoscalar particle, used in meson production and axion–photon conversion contexts.
Primary color	Color Science and Imaging	A selected set of colored lights or colorants whose mixtures define a target gamut under a specified additive, subtractive or other color-mixing model.
Primon Gas	Mathematical PhysicsStatistical Mechanics	In mathematical physics, the primon gas or Riemann gas discovered by Bernard Julia is a model illustrating correspondences between number theory and methods in quantum field theory, statistical mechanics and dynamical systems such as the Lee–Yang theorem.
Principal quantum number	Atomic PhysicsQuantum Mechanics	Principal quantum number denotes one of four quantum numbers which are assigned to each electron in an atom to describe that electron's state within atomic physics.
Principle of minimum energy	ThermodynamicsEquilibrium Extrema	A thermodynamic equilibrium principle stating that at fixed entropy and external parameters, a closed system's internal energy is minimized among nearby admissible states.
Process function	Thermodynamics	A thermodynamic quantity such as heat or work whose value depends on the path connecting equilibrium states rather than on the endpoints alone.
Projective superspace	Supersymmetric Field Theory	A superspace formalism that packages four-dimensional N=2 supersymmetric multiplets as holomorphic objects over an auxiliary complex projective line.
Projector Augmented-Wave Method	Electronic StructureComputational Condensed Matter	The projector augmented wave method (PAW) is a technique used in ab initio electronic structure calculations.
Proper reference frame (flat spacetime)	Special Relativity	An accelerated coordinate frame in Minkowski spacetime built around an observer's worldline, proper time, and carried spatial axes.
Proton Emission	Nuclear PhysicsCharged Particle Radioactivity Near the Proton Drip Line	A nuclear decay channel in which a proton-unbound state becomes a daughter nucleus with mass and atomic numbers each reduced by one while an outgoing proton penetrates the Coulomb and centrifugal barriers.
Pseudoscalar	SymmetryMathematical Physics	A scalar-valued quantity unchanged by proper rotations but sign-reversing under spatial orientation reversal.
Pseudospectral time-domain method	Computational PhysicsWave Simulation	A wave-simulation method that computes spatial derivatives through global spectral transforms while advancing the field on a discrete time grid.
Pupil function	Fourier Optics	A complex-valued aperture-plane function describing the amplitude transmission and phase shift imposed by an optical imaging system.
Quadrupole Formula	Gravitational Wave TheoryGeneral Relativity	A leading-order general-relativistic relation linking far-field gravitational-wave strain to a source's changing mass quadrupole moment.
Quantum annealing	Quantum Optimization	A heuristic optimization process that encodes an objective in a problem Hamiltonian and varies quantum fluctuations so low-energy candidate states are preferentially reached.
Quantum Bus	Superconducting Circuits	A quantum bus is a shared coherent mediator that lets separate qubits exchange states or interact without direct coupling.
Quantum circuit	Quantum Information and Computation	A model of quantum computation that represents initialized quantum registers, ordered gates, measurements, classical control, and outputs as a finite acyclic operation network.
Quantum Complex Network	Quantum Information ScienceQuantum Network Science	A complex network in which vertices, links, or both carry quantum systems and resources, so topology and admissible quantum operations jointly determine correlations, connectivity, and communication capability.
Quantum Concentration	Statistical MechanicsQuantum Gases	Quantum Concentration is a recurring statistical mechanics, quantum gases identity in which particle density reaches the scale where mean spacing equals thermal de Broglie wavelength and quantum statistics become appreciable.
Quantum digital signature	Quantum Cryptography	A signature protocol using quantum-state distribution and measurement properties to provide message authentication, transferability and resistance to repudiation or forgery.
Quantum electrodynamics	Quantum Field Theory	In particle physics, quantum electrodynamics (QED) is the relativistic quantum field theory of electrodynamics.
Quantum Energy Teleportation	Quantum Energy TeleportationLocal Operations Classical Communication	A correlated-state protocol uses a classically communicated local measurement outcome to condition remote local extraction of energy unavailable by the corresponding unconditioned action.
Quantum Field Theory	Theoretical Physics	Quantum Field Theory is a recurring theoretical physics, particle physics, condensed-matter physics identity in which quantized fields supply relativistic models whose excitations behave as particles or quasiparticles.
Quantum Fisher information	Quantum Metrology	The quantum Fisher information is a central quantity in quantum metrology and is the quantum analogue of the classical Fisher information.
Quantum Illumination	Quantum OpticsQuantum Sensing	A target-detection protocol that sends one mode of an entangled signal–idler pair into a noisy region and jointly measures its return with the retained idler.
Quantum instrument	Quantum Information and FoundationsQuantum Measurement Theory	Model a quantum measurement as an outcome-indexed completely positive operation-valued measure that jointly determines classical outcome probabilities and conditional post-measurement quantum states.
Quantum jump	Quantum Physics	An abrupt transition of a quantum system between discrete states, observed through a sudden change in emitted, absorbed or monitored signal.
Quantum Jump Method	Computational Quantum PhysicsOpen Quantum System Simulation	Unravel a Markovian open-system master equation into stochastic pure-state trajectories with non-Hermitian evolution interrupted by random jumps, then recover density-operator observables by ensemble averaging.
Quantum key distribution	Quantum Cryptography	A cryptographic key-establishment method that encodes and measures quantum states so eavesdropping creates detectable disturbance, followed by authenticated classical reconciliation and privacy amplification.
Quantum LC circuit	Quantum CircuitsCircuit Quantum Electrodynamics	and that is the main problem of the quantum LC circuit: energies stored on capacitance and inductance are not equal to the ground state energy of the quantum oscillator.
Quantum logic	Quantum Foundations	A nonclassical propositional structure modeled on the lattice of closed subspaces or projection operators associated with quantum measurements.
Quantum master equation	Quantum DynamicsOpen Quantum Systems	A differential equation for an open quantum system's reduced density operator, describing coherent evolution together with environment-induced dissipation, decoherence or memory effects.
Quantum Measurement	Quantum Mechanics	An outcome-producing quantum operation in which a state and measurement jointly determine Born-rule probabilities and generally induce an outcome-conditioned change of state.
Quantum number	Quantum MechanicsState Classification	A discrete or continuous label for an allowed quantum state, usually tied to eigenvalues of commuting observables or symmetry representations.
Quantum Operator	Quantum Mechanics	A quantum operator is a linear operator on a quantum state space, or between specified quantum spaces, whose domain, adjoint properties, algebra, and action represent an observable, symmetry, transformation, dynamical generator, measurement component, or information-processing gate.
Quantum Point Contact	Mesoscopic Physics	A short, narrow electronic constriction whose transverse dimensions are comparable to carrier wavelength, so transport proceeds through a small tunable set of quantum modes and ballistic conductance develops approximately quantized plateaus under suitable low-scattering conditions.
Quantum Rotation Operator	Quantum MechanicsAngular Momentum and Symmetry	Represent a physical spatial rotation on a quantum Hilbert space by a unitary operator generated by total angular momentum, preserving rotation composition while exposing the SO(3)/SU(2) distinction.
Quantum State	Quantum MechanicsQuantum Foundations	A normalized mathematical representation of a quantum system's preparation that determines outcome probabilities for admissible measurements under the theory's measurement rules.
Quantum state purification	Quantum Information Theory	In quantum information theory, quantum state purification refers to the process of representing a mixed state as a pure quantum state of higher-dimensional Hilbert space.
Quantum Stochastic Calculus	Quantum ProbabilityOpen Quantum Systems	An operator-valued stochastic integration calculus whose adapted quantum-noise increments obey an order-sensitive Itô product rule.
Quantum Triviality	Quantum Field TheoryStatistical Field Theory	A model-scoped conclusion that an interacting regulated field model loses its interaction along a specified continuum-limit trajectory, with the claim limited by its proof or numerical evidence.
Quantum Walk	Quantum InformationQuantum Dynamics	Coherent propagation of quantum amplitudes across graph positions by a unitary rule, with interference shaping measured location probabilities.
Quantum Zeno Effect	Quantum MechanicsQuantum Measurement	A quantum transition is inhibited when frequent effective interrogation confines evolution to a distinguished state or subspace.
Quantum-State Purity	Quantum MechanicsQuantum Information	The quadratic-trace measure of how concentrated a quantum density operator's eigenvalues are.
Quasi-Harmonic Approximation	Solid State Physics	The quasi-harmonic approximation computes crystal free energy from harmonic phonons whose frequencies depend on a global static constraint, commonly volume.
Quasi-Periodic Oscillation	X Ray Astronomy and Compact Object AstrophysicsRapid X Ray Variability	An astrophysical quasi-periodic oscillation is a statistically supported, finite-width peak in an accreting compact object's variability power spectrum, characterized without presuming one physical mechanism.
Quasisymmetry	Stellarator Plasma Confinement	A continuous symmetry of stellarator magnetic-field strength that yields an approximately conserved particle quantity and improved confinement despite asymmetric geometry.
R-symmetry	Supersymmetric Quantum Field Theory	Act by automorphisms on a supersymmetry algebra's supercharges while respecting spacetime symmetry, organizing multiplets, interactions, selection rules, and symmetry breaking.
Radiant Intensity	RadiometryOptics	In radiometry, radiant intensity is the radiant flux emitted, reflected, transmitted or received, per unit solid angle, and spectral intensity is the radiant intensity per unit frequency or wavelength, depending on whether the spectrum is taken as a function of frequency or of wavelength.
Radiation	ElectromagnetismRadiation Physics	In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium.
Radiation angle	Fiber OpticsOptics	In fiber optics, the radiation angle is half the vertex angle of the cone of light emitted at the exit face of an optical fiber.
Random Quantum Circuit	Random Quantum DynamicsQuantum Information	An ensemble of quantum circuits defined by a probability law over local gates, placements, or measurements, used to study statistical properties of quantum dynamics and outputs.
Random-Phase Approximation	Quantum Many Body TheoryCondensed Matter Physics	A many-body approximation that couples selected linearized particle-hole fluctuations to obtain collective response or correlation contributions.
Randomized Benchmarking	Quantum Information ScienceQuantum Control Characterization	A scalable quantum-control characterization protocol that estimates an average gate-error parameter from survival-probability decay across random, length-varying gate sequences followed by a recovery operation.
Rankine Scale	Temperature MetrologyThermodynamics	An absolute thermodynamic-temperature scale whose zero is absolute zero and whose degree interval equals one Fahrenheit degree (1 °R = 5/9 K).
Rankine vortex	Fluid DynamicsVortex Dynamics	The Rankine vortex is a simple mathematical model of a vortex in a viscous fluid.
Reduced Dynamics	Quantum PhysicsOpen Quantum Systems	The effective evolution of an open quantum subsystem obtained by evolving system and environment jointly and tracing out the environment, yielding a reduced-state trajectory and, when preparation assumptions permit, a dynamical map on system states.
Reduction criterion	Quantum InformationEntanglement and Separability Criteria	Certify a necessary condition for bipartite separability by requiring both reduced-state operators tensored with identity minus the joint density operator to remain positive semidefinite.
Reflected entropy	Quantum Information	A mixed-state correlation measure obtained by canonically purifying a bipartite density operator and taking entanglement entropy across the reflected subsystem split.
Reflection (Physics)	Wave PhysicsOptics	The generation of a wave component that returns into the incident medium when boundary conditions at an interface or obstacle redirect part of the incoming field.
Refraction	Wave Physics	The change in direction and wavelength of a wave caused by a change in propagation speed across space or between media.
Regge calculus	Mathematical Relativity	A discretization of general relativity that represents spacetime by a simplicial complex with curvature concentrated in deficit angles on codimension-two hinges.
Relative Permeability	Porous Media FlowMultiphase Constitutive Modeling	The phase-specific, dimensionless constitutive factor that reduces or modifies a porous medium's absolute permeability under multiphase occupancy, conditional on saturation, wettability, displacement history, and normalization convention.
Relativistic electromagnetism	Classical Field Theory	The unified spacetime formulation in which electric and magnetic fields are observer-dependent components of one electromagnetic field tensor.
Relativistic heat conduction	Relativistic Continuum Physics	A family of continuum models for thermal transport whose signals remain causal and stable within relativistic light cones.
Relativity of simultaneity	Special Relativity	The special-relativistic result that spatially separated events judged simultaneous in one inertial frame are generally assigned different times in another frame moving relative to it.
Reptation Monte Carlo	Computational Quantum Physics	A projector quantum Monte Carlo method that samples whole imaginary-time paths by extending one end and deleting the other, resembling polymer reptation.
Residual property (physics)	ThermodynamicsReal Fluid Departure Functions	A thermodynamic departure function equal to a real-fluid property minus the corresponding ideal-gas property at the same temperature, density or volume, and composition.
Rest frame	Special Relativity	An inertial reference frame in which the specified particle or system has zero spatial momentum or remains at fixed spatial coordinates.
Retrograde condensation	ThermodynamicsPhase Equilibria	Retrograde condensation occurs when gas in a tube is compressed beyond the point of condensation with the effect that the liquid evaporates again.
Reverberation	Acoustics	Persistence and gradual decay of sound caused by a dense succession of reflections arriving after the direct sound.
Reverse Diffusion	Transport Physics	Uphill transport in which a component moves toward higher concentration during an unstable or coupled mixture evolution because the chemical-potential gradient, rather than concentration gradient alone, drives flux.
Reversible reference system propagation algorithm	Computational PhysicsMolecular Dynamics Integration	Integrate molecular dynamics with a symmetric multiple-time-step factorization that evaluates fast force components frequently and slow components less often while preserving time reversibility.
Reynolds Number	Fluid Dynamics	Compare inertial transport with viscous momentum diffusion in a flow through the dimensionless ratio Re=ρUL/μ=UL/ν, using geometry-specific characteristic scales.
Right-Hand Rule	ElectromagnetismVector Orientation Conventions	In mathematics and physics, the right-hand rule is a convention and a mnemonic utilized to define the orientation of axes in three-dimensional space and to determine the direction of the cross product of two vectors, as well as to establish the direction of the force on a current-carrying conductor in a magnetic field.
Rigid rotor	Rotational Dynamics	An idealized rotating-body model in which mass distribution and internal distances remain fixed while orientation changes.
Rigidity Theory (Physics)	Network GlassesTopological Constraint Theory	Physical rigidity theory compares mechanical constraints with available motions in disordered networks to locate flexible, isostatic and rigid structure.
Rindler Coordinates	Relativity	Chart a wedge of flat Minkowski spacetime with coordinates adapted to uniformly accelerated observers, exposing their hyperbolic worldlines and observer-dependent horizon.
Ronchi test	OpticsOptical Testing	An optical figure test that samples light returned from a mirror through a periodic grating and compares the resulting band pattern with that predicted for the desired surface and test geometry.
Root-mean-square speed	Kinetic Theory of GasesStatistical Mechanics	The "root mean square speed" v_\text{rms} is the square root of the mean square speed, corresponding to the speed of a particle with average kinetic energy, setting b = \frac{1}{2a^2} = \frac{m}{2k_\text{B}T} : \begin{align}.
Rotating Black Hole	General RelativityGravitation	A black hole with nonzero angular momentum, modeled in stationary asymptotically flat general relativity by the Kerr family or, with electric charge, the Kerr–Newman family.
Rouse model	Polymer Physics	A coarse-grained polymer-dynamics model representing an ideal chain as Brownian beads linked by harmonic springs without hydrodynamic or excluded-volume interactions.
Runaway electrons	Plasma PhysicsFusion Plasmas	The term runaway electrons (RE) is used to denote electrons that undergo free fall acceleration into the realm of relativistic particles.
Rutherford model	Atomic PhysicsAtomic Models	The Rutherford model is a name for the concept that an atom contains a compact nucleus.
Sakuma–Hattori equation	Radiation ThermometryThermal RadiationRadiometry	In physics, the Sakuma–Hattori equation is a mathematical model for predicting the amount of thermal radiation, radiometric flux or radiometric power emitted from a perfect blackbody or received by a thermal radiation detector.
Saturation Vapor Pressure	Phase Equilibrium ThermodynamicsThermodynamics	The equilibrium partial pressure of water vapor above a flat surface of liquid water or ice at a stated temperature, used as the reference against which atmospheric moisture and saturation are expressed.
Scalar field	Mathematical Physics	A function assigning one scalar quantity to every point of a space or spacetime region, invariant under coordinate changes appropriate to a scalar.
Scalar field theory	Quantum Field TheoryClassical Field Theory	The most basic scalar field theory is the linear theory.
Scaling Dimension	Quantum Field TheoryConformal Field TheoryRenormalization Group	If the quantum field theory is scale invariant, scaling dimensions of operators are fixed numbers, otherwise they are functions of the distance scale.
Scattering	Scattering TheoryWave Propagation	A physical process in which incident particles or radiation are redirected or otherwise redistributed by interactions with localized or distributed inhomogeneities.
Scattering Length	Quantum Scattering	A signed length encoding the leading zero-energy s-wave response of a specified short-range elastic scattering channel.
Scheutjens–Fleer theory	Polymer PhysicsInterfacial Polymer Models	A lattice self-consistent-field framework for computing equilibrium segment-density profiles of polymers near interfaces under incompressibility and mean-field interaction assumptions.
Schlieren Imaging	Schlieren Optics	An optical imaging method that turns line-of-sight refractive-index-gradient deflections in a transparent medium into directional brightness contrast at a partially cut-off source image.
Schottky–Mott Rule	Semiconductor PhysicsMetal Semiconductor Junctions	The Schottky–Mott rule predicts metal–semiconductor barrier height from the metal work function and semiconductor electron affinity, with Fermi-level pinning as a known boundary.
Schrödinger Equation	—	Evolves a quantum state through a Hamiltonian generator and, in stationary settings, selects energy eigenstates through the corresponding eigenvalue equation.
Schwarzschild Metric	—	The one-mass-parameter, static and spherically symmetric vacuum spacetime geometry for zero charge, spin, and cosmological constant.
Schwarzschild Radius	General RelativityBlack Hole Physics	The mass-derived areal length 2GM/c², used as the horizon radius of an ideal nonrotating uncharged black hole and as a compactness scale for spherical bodies.
Second sound	Condensed Matter Physics	Wave-like propagation of temperature or entropy disturbances in a material where heat transport becomes hydrodynamic rather than diffusive.
Secondary emission	Surface PhysicsParticle Emission	The release of additional particles, usually electrons, from a material after energetic primary particles or radiation deposit energy at or near its surface.
Seeding (fluid dynamics)	Experimental Fluid DynamicsFlow Visualization and Particle Tracking	Introduce observable tracer particles into a flow so their motion can represent local fluid motion for visualization or velocity measurement, subject to fidelity and optical-bias constraints.
Seismic anisotropy	GeophysicsSeismology and Elasticity	Directional dependence of seismic-wave speed, polarization, or attenuation at a point, represented through elastic symmetry and used to infer layering, cracks, stress, and mantle fabric.
Seismic Inversion	GeophysicsSeismic Inversion	An inverse procedure that estimates subsurface physical-property models from seismic observations by combining a forward wave or convolutional model, prior constraints, an objective function, and uncertainty assessment.
Self-propelled particles	Active Matter PhysicsStatistical Physics	Self-propelled particles are active-matter model agents that consume or receive energy locally to generate persistent motion, producing collective states through interaction, noise, and environmental coupling.
Self-Propulsion	Active MatterLocomotion Physics	Self-propulsion is the autonomous displacement of nano-, micro- and macroscopic natural and artificial objects, containing their own means of motion.
Semi-empirical mass formula	Nuclear PhysicsLiquid Drop Model	In nuclear physics, the semi-empirical mass formula (SEMF; sometimes also called the Weizsäcker formula, Bethe–Weizsäcker formula, or Bethe–Weizsäcker mass formula to distinguish it from the Bethe–Weizsäcker process) is used to approximate the mass of an atomic nucleus from its number of protons and neutrons.
Semiconductor X-ray Photon-Counting Readout	Radiation DetectionX Ray Imaging	Resolve individual X-ray interactions in a semiconductor detector into threshold-accepted electrical events and accumulate their counts.
Semilinear response	Mesoscopic and Nonequilibrium Physics	A mesoscopic response framework in which energy absorption depends on the connected percolation of driven transitions through a sparse network in energy space rather than on their simple arithmetic average.
Sensible heat	ThermodynamicsHeat Transfer	Sensible heat is heat exchanged by a body or thermodynamic system in which the exchange of heat changes the temperature of the body or system, and some macroscopic variables of the body or system, but leaves unchanged certain other macroscopic variables of the body or system, such as volume or pressure.
Shape of the Universe	Physical Cosmology	Characterize cosmic space by separating its local curvature from its global topology, scale, connectedness, and boundary conditions under a stated cosmological model.
Shockley–Ramo theorem	ElectrostaticsRadiation Detectors	The Shockley–Ramo theorem is a method for calculating the electric current induced by a charge moving in the vicinity of an electrode.
Shortcut model	Network Statistical Mechanics	A random-network model that interpolates between regular lattices of neighboring integer dimensions by adding structured long-range shortcut layers.
Shortcuts to adiabaticity	Quantum Control	Control constructions that reproduce a target adiabatic evolution or endpoint in substantially shorter time.
SIC-POVM	Finite Dimensional Quantum TheoryQuantum State Tomography	A finite quantum measurement-effect family with d-squared rank-one outcomes and equal pairwise overlaps that makes state reconstruction possible from ideal outcome probabilities.
Simon–Glatzel equation	High Pressure Thermodynamics	An empirical power-law correlation relating a solid’s melting temperature to pressure through fitted reference pressure and exponent parameters.
Simple Wave	Hyperbolic Conservation LawsGas Dynamics	A simple wave is a smooth, one-characteristic-family region of a hyperbolic flow in which state changes while the other relevant invariant stays fixed.
Six-state protocol	Quantum CryptographyQuantum Key Distribution Protocols	Distribute quantum-key bits by randomly preparing and measuring qubits in three mutually unbiased bases, then sift, estimate errors, reconcile, and privacy-amplify the retained data.
Slater Determinant	Many Body Quantum Mechanics	Construct an antisymmetric many-fermion wavefunction as a determinant of occupied one-particle orbitals, making exchange-sign reversal automatic.
Slip Ratio (Gas–Liquid Flow)	Fluid MechanicsGas Liquid Two Phase Flow	The dimensionless ratio of gas to liquid phase-intrinsic velocity in a specified two-phase flow, distinguishing relative phase motion from equal-velocity flow.
Solar Jet	Solar PhysicsSolar Atmospheric Transients	A transient, narrow solar-atmospheric outflow in which magnetic restructuring releases stored energy, heats and accelerates plasma, and guides the ejecta along an elongated magnetic channel from a compact base.
Solar Neutrino Problem	Neutrino PhysicsParticle Astrophysics	The Nobel Committee for Physics, however, erred in mentioning neutrino oscillations in regard to the SNO Experiment: for the high-energy solar neutrinos observed in that experiment, it is not neutrino oscillations, but rather the Mikheyev–Smirnov–Wolfenstein effect, that produced the observed results.
Sommerfeld effect	Nonlinear MechanicsNonlinear Vibrations	Sommerfeld effect denotes mechanical phenomenon within nonlinear mechanics.
Sound from Ultrasound	Nonlinear AcousticsParametric Arrays	Sound from ultrasound refers to the generation of audible sound from modulated ultrasound.
Sound pressure	Acoustics	In acoustics, sound pressure or acoustic pressure is the pressure difference between the ambient (average or equilibrium) atmospheric pressure and that of a sound wave.
Spacetime	Relativity	The relativistic four-dimensional event framework whose local Lorentzian geometry distinguishes interval and causal directions, whether flat or curved.
Spatial frequency	Fourier Analysis Optics and Image Science	The rate at which a periodic or sinusoidal component repeats per unit distance, represented by reciprocal wavelength or angular wavenumber under an explicit cycles-versus-radians convention.
Special conformal transformation	Conformal Geometry and Mathematical Physics	A conformal map obtained by composing inversion, translation, and inversion, represented in Euclidean or Minkowski coordinates by a characteristic fractional transformation.
Special relativity	Relativistic Physics	A physical theory of spacetime in inertial frames based on invariant physical laws and invariant vacuum light speed, with events related by Lorentz transformations and spacetime interval rather than Galilean absolute time.
Specific quantity	MetrologyThermodynamics	An intensive physical quantity expressed per unit mass of a system.
Spectral line ratios	Plasma SpectroscopyAtomic Physics	The analysis of line intensity ratios is an important tool to obtain information about laboratory and space plasmas.
Spectral phase interferometry for direct electric-field reconstruction	Ultrafast OpticsPulse Metrology	An ultrashort-pulse characterization technique that retrieves spectral phase from interference between two frequency-sheared replicas and combines it with the measured spectrum to reconstruct the electric field.
Sphere of Influence (Astrodynamics)	AstrodynamicsCelestial Mechanics	An approximate boundary around an orbiting body where a patched-conic trajectory switches between secondary-centered and primary-centered two-body models.
Spin Diffusion	Solid State NmrSpin Dynamics	Spin diffusion is the transfer of spin order through coupled sites in a solid without corresponding movement of the nuclei.
Spin Hall Effect	Spintronics	Spin–orbit coupling converts longitudinal charge transport into a transverse spin current and opposite edge spin accumulation, with the reciprocal inverse effect converting spin transport into a charge response.
Spin network	Mathematical Physics	A labeled graph whose edges carry group representations and vertices carry invariant intertwiners, representing gauge-invariant quantum states or tensor contractions.
Spin squeezing	Quantum Information and Metrology	A collective quantum-spin state or process that reduces uncertainty in one transverse angular-momentum component below a coherent-spin benchmark while increasing conjugate uncertainty and often generating useful entanglement.
Spin tensor	Relativistic Field TheoryAngular Momentum Currents	An antisymmetric relativistic tensor or tensor current representing intrinsic angular-momentum density separately from orbital angular momentum in spacetime field descriptions.
Spin-exchange	Quantum Collision PhysicsAtomic Physics	In quantum mechanics, spin-exchange is an interaction process between two particles mediated by an exchange interaction.
Spontaneous process	Thermodynamics	A thermodynamic change that proceeds in its favored direction without continuous external driving because the relevant free energy decreases under stated constraints.
Squirmer	Microswimmer HydrodynamicsLow Reynolds Number Flow	A spherical low-Reynolds-number swimmer model that uses active surface velocity, Stokes flow and free-swimming force balance to connect propulsion to motion and disturbance flow.
Stability of the Second Kind	Statistical MechanicsMany Body Theory	A uniform lower bound proportional to particle count for a specified many-particle energy across all admissible system sizes.
Stabilizer code	Quantum Error CorrectionQuantum Information	A code whose space is the simultaneous positive eigenspace of an abelian Pauli subgroup.
Stable Yang–Mills–Higgs Pair	Gauge TheoryVariational Analysis of Yang Mills Higgs Functionals	A critical gauge–Higgs configuration whose specified Yang–Mills–Higgs functional has no negative second-variation direction in the admissible perturbation space after gauge redundancy is removed.
Standard Gravitational Parameter	Celestial Mechanics	Represent a body's Newtonian gravitational strength by the combined quantity \(\mu=GM\), the coefficient that orbital observations estimate directly and that enters two-body acceleration and orbit equations more precisely than separately inferred \(G\) and mass.
Star formation	Astrophysics	The gravitational collapse and fragmentation of dense interstellar gas into protostars that accrete and evolve toward stellar objects.
Starting vortex	Fluid Dynamics	The shed vortex formed near an airfoil’s trailing edge as circulation builds during acceleration from rest.
State function	Thermodynamics	A thermodynamic property determined only by the system’s equilibrium state, so its change is independent of the path between two states.
State-merging	Quantum Information	A quantum-information protocol that transfers one share of a joint state to a receiver who already holds correlated side information, at entanglement cost given by conditional quantum entropy.
Statistical field theory	Theoretical PhysicsStatistical Mechanics and Field Theory	Represent a many-body statistical system by fluctuating field configurations weighted by an effective energy or action, enabling correlation, scaling, path-integral, and renormalization analysis.
Steane Code	Quantum Error CorrectionStabilizer CodesQuantum Information	A [[7,1,3]] CSS stabilizer code derived from the Hamming code that corrects any single-qubit error.
STED microscopy	Super Resolution MicroscopyOptics	Stimulated emission depletion (STED) microscopy is one of the techniques that make up super-resolution microscopy.
Stefan adhesion	Fluid Mechanics	The viscous normal force resisting separation or approach of parallel surfaces with a thin Newtonian fluid layer between them.
Stimulated Raman Adiabatic Passage	Atomic Molecular Optical PhysicsQuantum Control	Coherent quantum-state population transfer by adiabatically following a dark path formed by overlapping target-side-before-initial-side Raman couplings, suppressing occupation of a shared intermediate level.
Stochastic quantization	Mathematical PhysicsEuclidean Quantum Field Theory	Represent a Euclidean quantum field measure as the stationary limit of an auxiliary-time stochastic process, allowing field correlation functions to be obtained as equilibrium stochastic averages.
Stochastic resonance	Statistical PhysicsNonlinear Dynamics	Stochastic resonance is also closely related to the concept of dithering in signal analysis, although how similar or how different the two concepts are depends on the particular definition considered.
Stokes wave	Fluid DynamicsWater Waves	A periodic nonlinear surface wave modeled with amplitude-dependent Stokes expansions of fluid motion.
Stokes's law	Fluid Dynamics	{\vec F}_{\rm d} is the frictional force – known as Stokes's drag – acting on the interface between the fluid and the particle (newtons, kg m s −2 ).
Stoneley wave	ElastodynamicsSolid Solid Interface Waves	Propagate an elastic interface mode along the bonded boundary between two solid media, with displacement concentrated near the interface and decaying into both adjoining materials.
Strain Localisation	Physical SciencesContinuum Mechanics	Distributed deformation becomes unstable and concentrates into a narrow band because local deformation changes the material or geometry in a way that attracts still more strain, creating a self-focusing path toward necking, shear banding, faulting, or breakup.
Stress space	Continuum MechanicsPlasticity and Yield Surfaces	Represent a symmetric stress tensor by its three principal stresses in Haigh–Westergaard coordinates, turning rotationally invariant yield criteria into surfaces decomposed by hydrostatic pressure, deviatoric radius, and Lode angle.
Stress triaxiality	Continuum Mechanics	The dimensionless ratio of mean hydrostatic stress to von Mises equivalent stress, characterizing the pressure-like component of a multiaxial stress state.
Stress–energy–momentum pseudotensor	General Relativity	In the theory of general relativity, a stress–energy–momentum pseudotensor, such as the Landau–Lifshitz pseudotensor, is an extension of the non-gravitational stress–energy tensor that incorporates the energy–momentum of gravity.
String duality	Theoretical PhysicsString Theory Dualities	Relate apparently different string-theory descriptions as equivalent formulations of the same physics, mapping spectra, couplings, compactification data, and observables across weak/strong, large/small, or geometric/nongeometric regimes.
String theory	Theoretical PhysicsQuantum Gravity	A quantum-gravity framework in which fundamental excitations are one-dimensional strings whose vibrational states and interactions generate particle species, forces and spacetime-dependent spectra.
Stuart Number	Magnetohydrodynamics	The Stuart number compares characteristic magnetic Lorentz forcing with fluid inertia in an electrically conducting flow.
Substance theory	Metaphysics	An ontological theory in which enduring particulars or substrata bear properties while remaining distinct from those properties.
Subterranean Rumbling	SeismoacousticsAudible Earthquake Acoustics	Recognize an audible earthquake sound when arriving seismic motion makes the local ground or nearby structures radiate pressure waves into the air, distinguishing a ground-to-air acoustic conversion from the felt shaking, distant infrasound, and objects merely rattling.
Sum rules (quantum field theory)	Quantum Field TheoryParticle Physics	A quantum-field-theory sum rule equates an integral over a dynamical quantity, such as a spectral or structure function, with a static property such as charge or mass.
Superdense coding	Quantum Information	A quantum communication protocol transmitting two classical bits by sending one qubit when sender and receiver share an entangled pair.
Superradiant phase transition	Quantum Optics	A collective quantum phase transition from a weakly excited state to one with macroscopic coherent occupation of a radiation mode and correlated emitter polarization.
Superstructure (condensed matter)	CrystallographySurface ScienceMagnetic Structure	A superstructure is a commensurately enlarged repeat of chemical, atomic, surface, or magnetic order relative to a specified simpler parent structure.
Supersymmetric WKB approximation	Quantum MechanicsSemiclassical Approximations	A semiclassical quantization method that applies WKB reasoning to a supersymmetric quantum-mechanical superpotential and is exact for many shape-invariant potentials.
Supersymmetry	Quantum Field TheoryQuantum Mechanics	A graded quantum symmetry whose odd supercharges connect even and odd sectors and close on an even dynamical generator.
Surface Code	Quantum Information ScienceTopological Quantum Error Correction	A two-dimensional topological stabilizer-code family that stores logical qubits in global boundary or homology classes while repeated local parity checks expose error-chain endpoints for decoding without directly measuring the logical state.
Surface Phonon	Surface Lattice Dynamics	Quantize a lattice-vibration normal mode whose amplitude or spectral identity is tied to a solid boundary, with in-plane wavevector, surface-modified force constants, and localization or resonance relative to projected bulk bands.
Su–Schrieffer–Heeger model	Condensed Matter PhysicsTopological Phases	In condensed matter physics, the Su–Schrieffer–Heeger (SSH) model or SSH chain is a one-dimensional lattice model that presents topological features.
Symmetry of diatomic molecules	Molecular PhysicsMolecular Spectroscopy	In addition to axial reflection symmetry, homonuclear diatomic molecules are symmetric with respect to inversion or reflection through any axis in the plane passing through the point of symmetry and perpendicular to the inter-nuclear axis.
Symplectic Structure	Classical Mechanics and Symplectic Geometry	Equip an even-dimensional manifold with a non-degenerate, closed 2-form ω that pairs each position with its conjugate momentum, so any smooth function becomes a flow and every such flow preserves ω exactly — forcing the Poisson bracket, Liouville's theorem, and the canonical-transformation test as consequences.
Synchronization of Chaos	Nonlinear Dynamics	Coupled or driven chaotic systems develop a stable relation—identical, phase, lag, generalized, or intermittent—despite sensitive dependence within each system.
Synchrotron function	Radiation PhysicsSpecial Functions	A pair of modified-Bessel-function kernels that encode the dimensionless frequency shape and polarization components of synchrotron-radiation spectra.
System of Measurement	MetrologyUnits of Measurement	A system of units of measurement, also known as a system of units or system of measurement, is a collection of units of measurement and rules relating them to each other.
Tadpole diagram	Quantum Field TheoryPerturbation TheoryQuantum Many Body Theory	A perturbative loop graph or loop subgraph attached to the rest of a Feynman graph through one line, with its physical contribution set by the surrounding calculation.
Tauc–Lorentz model	Optical PhysicsDielectric Function Models	The Tauc–Lorentz model is a mathematical formula for the frequency dependence of the complex-valued relative permittivity, sometimes referred to as the dielectric function.
Taylor Dispersion	Fluid Dynamics	Enhanced long-time axial tracer spreading from nonuniform flow coupled to transverse diffusion.
Taylor–Culick flow	Fluid Dynamics	An idealized axisymmetric flow in a long closed-end cylinder with uniform injection through a porous sidewall and axial discharge toward the open end.
Tearing mode	Plasma Physics	A plasma instability in which a perturbed sheared magnetic field reconnects across a resonant layer, producing magnetic islands.
Temperley–Lieb Algebra	Mathematical PhysicsDiagram Algebras and Exactly Solvable Lattice Models	A parameterized associative algebra whose local generators obey the Temperley–Lieb relations, equivalently the planar algebra of noncrossing strand diagrams composed by stacking with every contractible loop evaluated as a scalar.
Tensor Network	Quantum Many Body PhysicsTensor Factorizations	A graph-structured factorization of a high-dimensional tensor into smaller tensors, with edges denoting contracted indices, open edges denoting free indices, and bond dimensions controlling capacity.
Terminal Velocity	Fluid DynamicsDynamics	A steady speed relative to a fluid at which sustained driving force is balanced by drag and any buoyancy, leaving zero acceleration.
Test particle	Theoretical Physics	An idealized probe whose selected property responds to a field while its own influence on the modeled system is treated as negligible.
Tetrad formalism	Differential Geometry and General Relativity	A formulation of pseudo-Riemannian geometry and general relativity using a local orthonormal frame field whose components relate tangent-space Lorentz indices to spacetime coordinate indices.
Theory of solar cells	Photovoltaics	The semiconductor-physics framework explaining how absorbed photons generate, separate and collect charge carriers as electrical power in a photovoltaic device.
Thermal Expansion	Thermophysical Properties	Relate a material body's change in length, area, volume, or strain to temperature under stated mechanical and thermodynamic conditions through expansion coefficients that may vary by direction and state.
Thermal Quantum Field Theory	Theoretical PhysicsFinite Temperature Quantum Field Theory	A family of quantum-field-theoretic formalisms that replaces vacuum expectation values with traces over thermal statistical states, encoding temperature through density operators, imaginary-time boundary conditions, or real-time contours.
Thermoacoustic instability	ThermoacousticsCombustion Acoustics	Acoustic motion can modulate a heat source and receive phase-timed energy back, producing self-excited oscillation when growth is available on a linear or finite-amplitude path.
Thermoacoustics	Thermoacoustics	Couple oscillatory pressure and gas motion to phase-sensitive heat exchange so a temperature gradient can generate acoustic power or acoustic power can pump heat.
Thermodynamic Diagram	ThermodynamicsThermodynamic Representation	A thermodynamic diagram maps a specified system's properties into coordinates, paths, and regions so states or changes can be read under the chart's physical assumptions.
Thermodynamic process	Thermodynamics	A transformation carrying a thermodynamic system between states through a specified path of heat, work and matter exchange.
Thermodynamic square	Thermodynamics	Guggenheim) or Born square) is a mnemonic diagram attributed to Max Born and used to help determine thermodynamic relations.
Thermodynamic System	ThermodynamicsStatistical Mechanics	A declared macroscopic region or body separated from its surroundings by a boundary whose permitted matter and energy transfers determine thermodynamic accounting and system class.
Thermodynamic Temperature	ThermodynamicsTemperature Metrology	The thermal state quantity T of a specified equilibrium system, independent of the thermometer or temperature scale used to assign its value.
Thermogravitational Cycle	ThermodynamicsThermodynamic Cycles	A thermodynamic cycle using weight, buoyancy, and hydrostatic pressure to compress and expand a working fluid between heat reservoirs.
Thomas–Fermi Screening	Condensed Matter PhysicsElectron Gas	A static local-response approximation that uses electronic compressibility and self-consistent electrostatics to describe screening of a weak charge perturbation.
Three-photon interference	Quantum Optics	Quantum interference among indistinguishable alternatives for three photons propagating through a multimode optical network.
Tidal tensor	GravitationGeneral Relativity	The tidal tensor is the spatial gradient of gravitational acceleration, equivalently the relevant Hessian of gravitational potential or curvature component, mapping an infinitesimal separation vector to relative acceleration.
Time Reversibility	Time Reversal SymmetryStatistical Mechanics	Invariance of a dynamical or stochastic law under reversal of temporal order together with the appropriate transformation of state variables.
Time-translation symmetry	Symmetries and Conservation LawsClassical Mechanics	Time-translation symmetry or temporal translation symmetry (TTS) is a mathematical transformation in physics that moves the times of events through a common interval.
Top-Down Cosmology	Quantum Cosmology	Assign quantum-cosmological amplitudes to alternative no-boundary histories conditional on a specified present observational situation, so the relevant past-history ensemble depends on the question asked rather than on one observer-independent classical history.
Topological quantum field theory	Mathematical PhysicsTopological Field Theory	A quantum field theory whose observables depend only on topological structure, mathematically formalized as a symmetric monoidal functor from a cobordism category to vector spaces or related algebraic categories.
Topological superconductor	Condensed Matter Physics	A superconducting phase with a nontrivial bulk topological invariant and protected boundary or defect excitations.
Torque-Driven Spin-Axis Precession	Rotational DynamicsGyroscopic Motion	An external reorienting torque makes a spinning body's angular-momentum and spin-axis direction turn around a reference direction rather than merely changing its spin speed.
Torricelli's Equation	KinematicsClassical Mechanics	In physics, Torricelli's equation, or Torricelli's formula, is an equation created by Evangelista Torricelli to find the final velocity of a moving object with constant acceleration along an axis (for example, the x axis) without having a known time interval.
Total absorption spectroscopy	Nuclear Spectroscopy	Total absorption spectroscopy measures the complete gamma cascade following beta decay with a near-total-absorption detector.
Traction (mechanics)	MechanicsTangential Contact Force	Describe tangential force transmitted through a contact patch, while distinguishing actual demanded force from the friction-limited traction capacity available before gross slip.
Trans-Planckian Problem	Quantum Fields in Curved Spacetime	The robustness problem that arises when an observable low-energy prediction is obtained by evolving relevant field modes through frequencies or wavelengths beyond the Planck regime where the effective theory used in the derivation is not warranted.
Transformation of the electromagnetic field	ElectromagnetismSpecial Relativity	In the geometric view, the electromagnetic field is a six-dimensional geometric object in spacetime as opposed to two interdependent, but separate, 3-vector fields in space and time.
Transient electromagnetics	Geophysics	A time-domain geophysical method that drives a pulsed primary electromagnetic field and measures the decay of induced subsurface currents to infer electrical structure.
Translation operator (quantum mechanics)	Quantum Mechanics	In quantum mechanics, a translation operator is defined as an operator which shifts particles and fields by a certain amount in a certain direction.
Translational symmetry	Symmetry and Physics	Invariance of an object, field, law or equation under every translation in a stated continuous group or under translations in a discrete lattice.
Transmission coefficient	Wave and Barrier Physics	A convention-dependent amplitude, intensity, probability, or power ratio quantifying how much of an incident wave or flux passes through a boundary or barrier.
Transparency and translucency	OpticsLight Transmission	Optical transmission properties distinguished by scattering: transparent media preserve image-forming direction through the material, while translucent media transmit light but diffuse spatial detail.
Transport integrals	Statistical Physics	A family of special integrals weighting powers of a dimensionless variable by a thermal occupation-response kernel in solid-state transport theory.
Transverse isotropy	Continuum MechanicsElasticity	A transversely isotropic (also known as polar anisotropic) material is one with physical properties that are symmetric about an axis that is normal to a plane of isotropy.
Twelvefold Way	—	A twelve-case enumeration framework classifying allocations by whether objects and boxes are distinguishable and whether the assignment is unrestricted, injective, or surjective.
Twilight	Atmospheric Optics	Twilight is lower-atmosphere and surface illumination by scattered sunlight while the Sun lies below the horizon.
Twin Paradox	Special Relativity	The twin paradox is a special-relativity thought experiment in which clocks reunite after different timelike worldlines and record unequal proper times, dissolving the apparent contradiction from reciprocal inertial time dilation.
Twistor space	Twistor Theory	A complex-geometric space whose points or subspaces encode conformal spacetime geometry, null directions and solutions of the twistor equation.
Two-Higgs-Doublet Model	Particle Physics	An electroweak extension with two scalar Higgs doublets, producing a mixed five-state scalar sector whose behavior depends on the potential, vacuum, and Yukawa assignments.
Two-point tensor	Continuum Mechanics	A tensor-like linear map whose indices transform in two different vector spaces, commonly connecting a material reference configuration with a current spatial configuration.
Udwadia–Kalaba Formulation	Analytical MechanicsConstrained Dynamics	An explicit constrained-mechanics method that corrects free acceleration by a mass-weighted pseudoinverse to satisfy feasible ideal acceleration constraints.
Umbrella sampling	Computational Statistical Physics	Biased importance sampling that restrains exploration along a coordinate and reweights observations to estimate otherwise rare regions.
Unified field theory	Fundamental Interaction Unification	Seek a single field-theoretic structure whose fields, symmetries, or geometric components recover interactions that otherwise appear as separate fundamental forces.
Unruh Effect	—	A uniformly accelerated detector in Minkowski vacuum responds thermally at temperature k_B T = hbar a/(2 pi c), revealing observer-dependent particle content in quantum field theory.
Unruh–DeWitt Detector	Quantum Field Theory in Curved SpacetimeRelativistic Quantum Information	An idealized local quantum detector coupled to a field along specified spacetime support, with field-dependent transitions or detector correlations as its response.
Vacuum Energy	Quantum Field Theory	The energy associated with the quantum-field vacuum state, whose absolute gravitational contribution is linked to cosmological-constant physics while differences and correlations underlie observable effects attributed to vacuum fluctuations.
Van der Pol oscillator	Nonlinear DynamicsElectrical Oscillators	In the study of dynamical systems, the van der Pol oscillator (named for Dutch physicist Balthasar van der Pol) is a non-conservative, oscillating system with non-linear damping.
Van der Waals Equation	—	A cubic equilibrium equation of state that corrects the ideal-gas law for finite molecular exclusion and mean-field attraction, producing real-fluid nonideality and a liquid-gas critical point.
Vanishing scalar invariant spacetime	General Relativity	A Lorentzian spacetime whose polynomial scalar curvature invariants of every derivative order all vanish despite possible nonzero curvature.
Verlet Integration	—	A second-order, time-reversible symplectic family for integrating Newtonian motion by staggered position, velocity, and force updates, prized for long-time geometric stability.
Violating cosmic censorship	General Relativity	The weak and the strong cosmic censorship hypotheses are two mathematical conjectures about the structure of gravitational singularities in the context of general relativity.
Virtual image	OpticsGeometrical Optics	A real image is the collection of focus points made by real converging rays, while a virtual image is the collection of focus points made by backward extensions of real diverging rays.
Vis-viva equation	Celestial MechanicsOrbital Mechanics	The Keplerian two-body relation v² = μ(2/r − 1/a) linking orbital speed, radius, and semimajor axis.
Viscous Stress Tensor	Fluid MechanicsSolid Mechanics	A model-specified, rate-dependent tensor contribution to continuum stress that can transmit traction and enter momentum balance.
Volume viscosity	Fluid DynamicsContinuum Mechanics	A fluid's dissipative resistance to volume-changing motion, distinct from shear viscosity and elastic compressibility.
Vortex Ring	Fluid Dynamics	A closed, ring-like concentration of vorticity in a fluid, with circulation and an induced velocity field, formed by more than one physical route.
Vorticity confinement	Computational Fluid DynamicsVortex Dominated Flow Simulation	A physics-based numerical model that adds a localized force to preserve or reconstruct concentrated vortices otherwise diffused by discretization.
Water retention on random surfaces	Percolation TheoryStatistical Physics	Water retention on random surfaces is the simulation of catching of water in ponds on a surface of cells of various heights on a regular array such as a square lattice, where water is rained down on every cell in the system.
Wave Packet	Wave Physics	Build a localised disturbance as a narrow-band superposition of plane waves so it has both a position and a spectrum, then read its centroid off the group velocity and its spreading off the curvature of the medium's dispersion relation.
Wave shoaling	Surface Gravity WavesCoastal Oceanography	Shallowing water can slow a surface wave's group propagation and concentrate its energy into greater wave height along a sufficiently conserved path.
Wave vector	Wave Propagation	Represent the spatial phase gradient of a plane or locally plane wave by a vector whose direction fixes constant-phase orientation and whose magnitude is angular wavenumber.
Wavefront	Wave PropagationOptics	A fixed-time locus of equal phase in a propagating physical wave field, whose shape follows the selected phase geometry.
Wavefront coding	Computational Optics	Wavefront coding combines pupil-plane phase modulation with computational deconvolution to extend an imaging system's depth of field.
Wavenumber-frequency diagram	Wave PhysicsSpectral Analysis	A spectral plot of temporal frequency against spatial wavenumber whose energy ridges reveal propagation direction, phase velocity, group velocity, and dispersion.
Whiteness (colorimetry)	ColorimetryColor ScienceOptics	A standardized colorimetric measure for how closely a near-white surface approaches a reference white under a specified illuminant and observer, calculated from tristimulus/chromaticity coordinates together with a tint value and valid only within defined near-white limits.
Widom Insertion Method	Statistical Mechanics	Estimate a component's excess chemical potential by averaging the Boltzmann factor of the energy change from hypothetical test-particle insertions into equilibrium configurations.
Widom Scaling	Critical PhenomenaStatistical Mechanics	A critical-phenomena hypothesis that treats singular free energy as a homogeneous scaling function, thereby relating critical exponents and collapsing near-critical behavior onto reduced variables.
Wigner D-Matrix	Quantum Angular Momentum	The spin-j unitary matrix of an SO(3) or SU(2) rotation in the spherical angular-momentum basis, with Euler-angle factorization exposing its reduced small-d functions.
Wigner rotation	Special Relativity	The spatial rotation component produced when non-collinear Lorentz boosts are composed.
Wigner Surmise	Nuclear Physics	Approximate unfolded nearest-neighbor level spacings in a random-matrix symmetry class by the exact small-matrix spacing law, capturing level repulsion with a simple normalized density.
Wigner–Weyl transform	Mathematical Physics	An invertible correspondence between phase-space functions and quantum operators that underlies the quasiprobability formulation of quantum mechanics.
Witten Index	Supersymmetric Quantum Mechanics	The Witten index is a fermion-parity-graded supersymmetric trace that cancels paired positive-energy states and counts zero modes with signs.
Wolf tone	Musical AcousticsCoupled String Body Resonance	An unstable, warbling note on a bowed-string or related resonant instrument produced when string excitation strongly couples to a nearby body resonance, causing competing vibration regimes rather than a steady fundamental.
Work (thermodynamics)	ThermodynamicsEnergy Transfer	Energy transferred across a thermodynamic system boundary through generalized macroscopic force–displacement interactions rather than through heat transfer or matter flow.
Work Output	Thermodynamics	Quantify energy transferred from a machine or thermodynamic system to its surroundings through an organized mechanical mode, evaluated over a declared process or cycle.
Yang–Mills Equations	—	The Yang–Mills equations require a connection's curvature to have zero gauge-covariant divergence, making the connection stationary for the Yang–Mills action.
Yang–Mills theory	Theoretical Physics	A non-Abelian gauge-field theory in which a connection on a principal bundle has curvature dynamics derived from the Yang–Mills action.
Zak phase	Condensed Matter Physics	The Berry phase accumulated by a Bloch band along a closed noncontractible loop through the Brillouin zone.
Zeeman effect	Atomic SpectroscopyAtomic Physics	The Zeeman effect () is the splitting of a spectral line into several components in the presence of a static magnetic field.
Zero-point energy	Quantum MechanicsQuantum Field Theory	Zero-point energy (ZPE) is the lowest possible energy that a quantum mechanical system may have.
Zoeppritz Equations	GeophysicsReflection Seismology	The exact plane-wave interface relations that use elastic boundary continuity to partition an incident P or S wave among reflected and transmitted P and S modes as a function of angle and material properties.
Μ(I) rheology	Granular PhysicsRheology	In granular mechanics, the \boldsymbol{\mu(I)} rheology is a rheological model for granular flows, describing the evolution of the macroscopic friction coefficient \mu as a function of the dimensionless quantity I called the Inertial number.