Quasithermal

Quasithermal is an adjective used in physics and related fields to describe a state or distribution that is broadly consistent with thermal equilibrium but exhibits departures due to incomplete thermalization, ongoing energy input, or non-equilibrium dynamics. A quasi-thermal description typically involves an effective temperature parameter that characterizes low-energy behavior while allowing non-thermal features at higher energies or in other degrees of freedom.

In space and plasma physics, quasi-thermal or quasi-Maxwellian distributions describe particle populations that are approximately Maxwellian at low energies but have non-Maxwellian tails; the kappa distribution is a widely used family that interpolates between a thermal core and a power-law tail, often regarded as quasi-thermal in practice.

In high-energy astrophysics and X-ray astronomy, quasi-thermal emission or quasi-thermal Comptonization refers to radiation produced by electrons that are nearly thermal but possess a non-thermal tail, producing spectra between purely thermal (blackbody-like) and fully non-thermal (power-law) shapes.

In laboratory and condensed-matter contexts, quasi-thermal states can arise when a system is coupled to a finite-temperature bath that implements slow relaxation, or under external driving that maintains a steady state with an effective temperature.

Important caveats: quasi-thermal does not imply true thermal equilibrium; the precise definition is field-specific; data interpretation often uses mixture models with a thermal component plus non-thermal component; thus, the term is descriptive rather than universal.