The Laws of Thermodynamics
Thermal equilibrium of two systems. If two isolated systems A and B are brought into contact with each other, then the complete system A + B eventually goes into a state of thermal equilibrium. In this case it is said that the systems A and B are in a state of thermal equilibrium with each other. Each of the systems A and B individually is also in a state of thermal equilibrium. This equilibrium will not be broken if we remove the contact between the systems, and then after a while restore it. Consequently, if the establishment of contact between the two systems A and B, which previously were isolated, does not lead to any changes, then we can assume that these systems are in thermal equilibrium with each other (A ~ B).