1. A state in which two objects, or an object and its surroundings, have the same temperature so that there is no exchange of heat energy between them. For example, a telescope mirror should ideally be in thermal equilibrium with its supports and with the atmosphere to prevent distortion of the mirror or the creation of air currents within the telescope’s tube.

2. A state in which the available energy of an object is distributed uniformly among all the possible forms of energy; also known as thermodynamic equilibrium. For example, deep inside a star the radiation field, the kinetic energy, the excitation energy, and the ionization levels will all have equal amounts of energy. Furthermore, all processes are in balance so that, for example, there will be as many ionizations of helium per second as there are recombinations of free electrons and helium ions. The condition of local thermodynamic equilibrium is often taken as an approximation when modelling stellar atmospheres.