Thermal equilibrium is a state of a system in which there is no net heat transfer between different parts of the system, and the temperatures throughout the system are constant and uniform. In other words, when two or more objects or systems are in thermal equilibrium, there is no temperature difference between them, and they are at the same temperature.

The concept of thermal equilibrium is based on the zeroth law of thermodynamics, which states that if two systems are separately in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.

When two objects at different temperatures are brought into contact or placed in close proximity to each other, heat will flow from the hotter object to the colder one until both reach the same temperature. Once they reach the same temperature, they are said to be in thermal equilibrium, and there is no further heat transfer between them.