When two bodies are in the the same temperature, they are said to be in thermal equilibrium with each other.

In Figure 1, if body \(A\) is in thermal equilibrium with body \(B\) and body \(B\) is in thermal equilibrium with body \(C\), body \(A\) and body \(C\) will also be in thermal equilibrium and this is called zeroth law of thermodynamics.