Question

What condition is necessary for two bodies that are in physical contact with each other to be in thermal equilibrium?

Answer 1

The Zeroeth law of thermodynamics introduces the concept of a statefunction namely temperature whose value determines the condition of thermal equilibrium.

Two (or more) systems having the same value of temperature are in thermal equilibrium with one another and any number of such systems being in diathemic contact with each other results in no net heat transfer from one system to another.