The Zeroth Law of Thermodynamics states that if two bodies are each in thermal equilibrium with some third body, then they are also in equilibrium with each other. Thermal equilibrium means that when two bodies are brought into contact with each other and separated by a barrier that is permeable to heat, there will be no transfer of heat from one to the other.

This says in essence that the three bodies are all the same temperature. James Clerk Maxwell put this perhaps more simply when he said, “All heat is of the same kind.” What is most important is that the Zeroth Law establishes that temperature is a fundamental and measurable property of matter.