In physics, the phrase thermal equilibrium is used sometimes in the common parlance of the ordinary language of physical discourse, and sometimes as a specialized technical term in thermodynamics.

As common parlance, the phrase refers to steady states of temperature, which may be spatial or temporal. The meaning varies from occasion to occasion, as with all ordinary language usages.

Thermal equilibrium as a technical term in thermodynamics can also be used in two senses. One sense is that of thermal equilibrium within a system for itself. The other sense is that of a relation between the respective physical states of two bodies. Thermal equilibrium in a system for itself means that the temperature within the system is spatially and temporally uniform. Thermal equilibrium as a relation between the physical states of two bodies means that there is actual or implied thermal connection between them, through a path that is permeable only to heat, and that no energy is transferred through that path. This technical sense is concerned with the theory of the definition of temperature.