Definition - What does Thermal Equilibrium mean?
Thermal Equilibrium is a state at which two substances in physical contact have no difference in their body temperatures, i.e., either the objects have same temperature or both objects are connected by a permeable barrier that does not allow the transfer of heat between the two objects. Thermal equilibrium is mainly associated with the laws of thermodynamics in physics and mechanics.
Petropedia explains Thermal Equilibrium
It is commonly observed that when various multiple objects come in contact with each other, they tend to transfer heat energy amongst each other. The object with high temperature transfers heat to the object with cold temperature. This way objects attain the same or constant temperature and are said to be in thermal equilibrium. Thermal equilibrium is subject to Zeroth Law of Thermodynamics. As per Zeroth Law of Thermodynamics, if the first object is in equilibrium with the second object and the second object is in equilibrium with the third object, then first and third objects will also be in thermal equilibrium. This means that when objects that are in thermal equilibrium with each other are brought in contact, there is no heat transfer or loss of heat between them.