Zeroth law of thermodynamics

The Zeroth Law of Thermodynamics states that if two thermodynamic systems A and B are in thermal equilibrium, and B and C are also in thermal equilibrium, then A and C are in thermal equilibrium. Thermal equilibrium means that if the systems are connected in some way, then heat would not be exchanged.

The unusual name of 'zeroth law' became necessary when this concept was found more fundamental and basic than the already existing laws of thermodynamics. The utility of this law is allowing systems to be defined by the empirical measurement of temperature.