Which of the following laws defines thermal equilibrium?

The concept of thermal equilibrium is defined by the zeroth law of thermodynamics. This law states that if two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other. This foundational principle allows us to define temperature in a consistent manner across different systems.

When two systems are at the same temperature and no heat flows between them, they are considered to be in thermal equilibrium. The zeroth law is fundamental because it establishes the basis for temperature measurement and the concept that temperature is a property that can be used to determine thermal equilibrium between different systems.

In the context of thermodynamic laws, the first law focuses on the conservation of energy, the second law deals with the direction of spontaneous processes and the concept of entropy, and the third law addresses the behavior of systems as they approach absolute zero. None of these laws specifically describe the condition of thermal equilibrium, which is why the zeroth law is the correct answer.