It is observed that a higher temperature object which is in contact with a lower temperature object will transfer heat to the lower temperature object. The objects will approach the same temperature, and in the absence of loss to other objects, they will then maintain a constant temperature. They are then said to be in thermal equilibrium.
Thermal equilibrium is the subject of the Zeroth Law of Thermodynamics .
It is so named because it logically precedesthe First and Second Laws of Thermodynamics.