If two systems are in thermal equilibrium with a third system then they must be in thermal equilibrium with each other.

This statement is called the Zeroth law of thermodynamics and forms the basis of concept of temperature. All these three systems can be said to possess a property that ensures their being in thermal equilibrium withone another. This property is known as temperature. We may, therefore, define the temperature of a system as the property that determines whether or not the system is in thermal equilibrium with the neighbouring systes. It is obvious that if two systems are not in thermal equilibrium, they will be at different temperatures.