The Zeroth Law of Thermodynamics and Temperature

The Zeroth Law of Thermodynamics states: if two systems, A and B, are separately found to be in thermal equilibrium with a third system, C, then A and B must also be in thermal equilibrium with each other. Two systems are said to be in thermal equilibrium when, on being placed in contact through a thermally conducting (diathermic) boundary, no net flow of heat occurs between them.

This law is called the "zeroth" law because it was recognized as a necessary, independent postulate only after the First, Second and Third Laws had already been named and numbered — yet logically it had to come before all of them, since it is what makes the entire concept of temperature rigorous in the first place. Without the Zeroth Law, there would be no logical basis for saying that two objects have "the same temperature" simply because a thermometer placed in contact with each of them independently gives the same reading. …