All articles starting with Q

Quasi-likelihood

Quasi-likelihood is a statistical method for estimating and testing model parameters from only the mean and variance of the data, without assuming a full probability distribution.

Oct 10, 2026

Quasi-market

A quasi-market is a system for delivering publicly funded services in which the state finances the service but independent providers compete, a term coined by Julian Le Grand in 1991.

Quasi-maximum likelihood estimation

Quasi-maximum likelihood estimation (QML, or pseudo maximum likelihood, PML) is a statistical method that maximizes a convenient likelihood even when the assumed distribution is wrong.

The quasi-Monte Carlo (QMC) method is a numerical integration technique that replaces random sample points with deterministic, evenly spread low-discrepancy points, so the integration error shrinks faster with sample size.

Quasi-Newton method

A quasi-Newton method, also called a variable metric method, is an optimization algorithm that minimizes a differentiable function by approximating the Hessian from successive gradients.

Quasi-orthogonal space-time block code

A quasi-orthogonal space-time block code (QOSTBC) is a space-time block code for multiple-antenna wireless transmission in which code matrix columns are only partly orthogonal, permitting a symbol rate of one with more than two transmit antennas.

Quasi-periodic eruption

A quasi-periodic eruption (QPE) is a repeating burst of soft X-ray radiation, lasting about an hour and recurring every few hours, produced near a galaxy's central massive black hole.

Quasi-periodic oscillation

A quasi-periodic oscillation (QPO) is a nearly periodic flickering of X-ray brightness from an accreting neutron star or black hole, first seen in early 1996.

Quasi-phase-matching

Quasi-phase-matching is a nonlinear optics technique that periodically reverses a crystal's nonlinear coefficient, keeping light frequency conversion efficient despite phase mismatch, typically by periodically poled crystals like lithium niobate.

Quasi-satellite

A quasi-satellite, or quasi-moon, is a small body that orbits the Sun in step with a planet, appearing to circle the planet without being bound to it.

Quasi-star

A quasi-star, or black hole star, is a hypothetical early-universe object with a black hole inside a massive gas envelope, proposed as a seed of supermassive black holes.

Quasi-stationarity in branching processes

Quasi-stationarity in branching processes studies quasi-stationary distributions on the non-extinct states of a branching process conditioned on survival, including the Yaglom limit for Galton–Watson processes and applications in ecology and epidemiology.

Quasi-symmetric stellarator

A quasi-symmetric stellarator is a stellarator whose magnetic field strength is approximately symmetric in one direction, reducing particle losses to tokamak-like levels; the HSX experiment first tested it.

Quasi-War

Quasi-War: undeclared naval war between the United States and France, 1798–1800, sparked by French seizures of American ships after the Jay Treaty and XYZ Affair; ended by the Convention of 1800 (Treaty of Mortefontaine).

Quasi-Z-source inverter

The quasi-Z-source inverter is a single-stage DC-to-AC converter that boosts and inverts in one stage, introduced by Anderson and Peng in 2008 for photovoltaic and fuel-cell uses.

Quasi-Zenith Satellite System (準天頂衛星システム)

The Quasi-Zenith Satellite System (準天頂衛星システム), QZSS, nicknamed Michibiki, is Japan's satellite positioning system, complementing GPS in the Asia-Oceania region; four-satellite service began in 2018, expanding toward seven satellites.

Quasi-zero-stiffness metamaterial vibration isolators

Quasi-zero-stiffness (QZS) metamaterial vibration isolators are periodic architected structures combining positive and negative stiffness elements to block low-frequency vibration while bearing heavy static loads, with tunable, programmable designs.

Quasicrystal

A quasicrystal, or quasiperiodic crystal, is an ordered solid whose atoms lack periodic repetition, giving sharp diffraction patterns with forbidden fivefold symmetries, first observed by Dan Shechtman in 1982.

Quasielastic neutron scattering

Quasielastic neutron scattering (QENS) is a neutron spectroscopy technique in which neutrons exchange tiny amounts of energy with a sample, revealing diffusive molecular motions on picosecond to nanosecond timescales.

Quasienergy

Quasienergy is the quantity that plays the role of energy for a quantum system with a Hamiltonian periodic in time, as described by Floquet theory.

Quasigroup

A quasigroup is a set with a binary operation in which division is always possible in both directions, like a group without needing associativity or an identity.

Quasimodo

Quasimodo is the deaf, hunchbacked bell-ringer of Victor Hugo's 1831 novel Notre-Dame de Paris (The Hunchback of Notre Dame), named for Quasimodo Sunday; he protects Esmeralda, kills Frollo, and symbolizes courage beneath ugliness.

Quasimoto

Quasimoto, or Lord Quas, is the rap alter ego of Madlib. The character debuted in 2000 with the album The Unseen, known for its high-pitched, helium-like voice.

Quasineutrality

Quasineutrality is the property of a plasma by which ion and electron densities are nearly equal, so the bulk carries no net charge except in thin sheaths near walls.

Quasinormal modes

Quasinormal modes (QNMs) are the characteristic damped oscillations of a perturbed black hole or compact star, like the tones of a struck bell, underlying black-hole spectroscopy.

Quasinormal modes of black holes

Quasinormal modes of black holes are damped oscillations of a perturbed black hole that dominate the ringdown after a merger, with frequencies encoding the hole's mass and spin.

Quasiparticle

A quasiparticle is an excitation of an interacting many-particle system that behaves like a single particle with well-defined energy and momentum, introduced by Lev Landau.

Quasiparticle interference

Quasiparticle interference (QPI) is the standing-wave pattern formed when electronic quasiparticles scatter from impurities or defects and interfere, letting a scanning tunneling microscope recover a material's electronic structure.

Quasiparticle poisoning

Quasiparticle poisoning is the degradation of quantum information in superconducting qubits, caused by quasiparticles tunneling across Josephson junctions, causing decoherence in transmons and flipping the encoded parity of topological qubits.