Question

What is Thermal equilibrium ​

Answer 1

Thermal equilibrium stands any substance / object/ body said to be in thermal equilibrium if there is no net flow of thermal energy (e.g.heat) between them and must be they are connected by path permeable to thermal energy or heat.

Answer 2

Suppose a body A and B are separated by a conducting wall. And a third system C is connected to both by an adiabatic(non conducting) wall. Now both the bodies A and B will eventually arrive at the same temparatures. Now the wall between them is replaced by adiabatic wall and the adiabatic with conducting. Now there will be no change in the temperature. This means that:-

If 2 bodies are in thermal equilibrium with a third body C then they are in thermal equilibrium with each other.