0th Law of Thermodynamics

What is the Zeroth Law of Thermodynamics?

Hint
The Zeroth law defines a property (temperature) and describes its behavior.
There exists for every thermodynamic system in equilibrium a property called temperature. Equality of temperature is a necessary and sufficient condition for thermal equilibrium.
There exists for every thermodynamic system in equilibrium a property called temperature. Equality of temperature is a necessary and sufficient condition for thermal equilibrium.