“If two systems are each in thermal equilibrium with a third system, then the two systems are also in thermal equilibrium with one another.”

This law was enunciated by R.H. Fowler in 1931 however since the first and second law already existed at that time it was designated as Zeroth law so that it precedes the first and second law to form a logical sequence.

If System A and System C, System B and System C are in thermal equilibrium with each other then system A and B is also in thermally equilibrium with each other.