What is the name of the law that establishes the concept of temperature?

What is the name of the law that establishes the concept of temperature? It is the zeroth law of thermodynamics.

The law states that if system A is in thermal equilibrium with system C, and system B is also in thermal equilibrium with system C, then A and B are in thermal equilibrium with each other. In practical terms, a thermometer can compare temperatures because systems sharing the same thermal equilibrium state have the same temperature.

This principle provides the foundation for defining temperature independently of heat flow or entropy. It explains why a thermometer can be placed in contact with an object, allowed to reach equilibrium, and then used to compare that object with another system.

The name can be confusing because the first, second, and third laws were formulated or recognized earlier. Physicists later realized that this foundational principle logically belonged before the first law, so it was named the “zeroth” law. Ralph H. Fowler is generally credited with introducing the term in the 1930s.

The first law concerns conservation of energy, the second law concerns entropy and the direction of natural processes, and the third law concerns behaviour near absolute zero. None of those supplies the same basic definition of thermal equilibrium and temperature.