Thermal equilibrium is a state in which two or more objects in thermal contact have reached the same temperature, resulting in no net heat flow between them. This occurs when the energy transferred as heat between the objects equals zero, meaning they are no longer exchanging thermal energy. At this point, all parts of the system have the same temperature, and the thermal energy is evenly distributed.
The concept is rooted in the second law of thermodynamics and is crucial for understanding heat transfer, thermodynamic processes, and the behavior of systems in statistical mechanics. In practical terms, thermal equilibrium is often achieved in everyday situations; for example, when a hot cup of coffee is left in a cooler room, it will eventually cool down to room temperature, reaching thermal equilibrium with its surroundings.