In physics, thermalization is the process by which a system of particles acquires some common temperature as a result of mutual collisions. The thermalized gas in a room quickly comes to equilibrium with the surrounding walls and objects, assuming they are at different temperatures. The concept of thermalization is important in statistical mechanics and thermodynamics.
A system can lose its thermal equilibrium if it gains or loses heat energy faster than it can dissipate that energy through collisions. In such a case, the system is said to be out of equilibrium or non-thermalizing. Non-thermal systems occur in nature, but are usually quite small and only exist for very short periods of time; examples include subatomic particle interactions, nuclear reactions, and astronomical events such as supernovae.