Zeroth law of Thermodynamics:
The zeroth law of Thermodynamics states that if two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other.
Also, another fundamental fact is that, when two bodies at different temperatures are brought into contact, after some time they attain a common temperature and they are said to be in thermal equilibrium.
The temperature is a crucial parameter in the subject of Thermodynamics. The temperature is what distinguishes Thermodynamics from other sciences.
Significance of zeroth law of Thermodynamics:
So, using the above two statements and if we replace the 'third body' with a thermometer, zeroth law can be redefined as two bodies are in thermal equilibrium if both have the same temperature reading on the thermometer. Hence, the zeroth law of Thermodynamics gives rise to the definition of temperature.