Equality of Temperature:
Two systems are having equal temperature if there are no changes in their properties when they are kept in thermal contact with each other.
Zeroth Law: Statement
When a body A is in thermal equilibrium with body B, and separately with body C, then B and C both will be in thermal equilibrium with each other. This is called as the zeroth law of thermodynamics.
This law forms basis for all the temperature measurement. The thermometer functions as body 'C' and compares unknown temperature of body 'A' with a known temperature of body 'B'
This law was given by R.H. Fowler in the year 1931. However, as the 1st and 2nd laws already existed at that time, it was selected as Zeroth law so that it precedes the 1st and 2nd laws to form a logical sequence.