When two bodies A and B are separately in thermal equilibrium with a third body C, they are in thermal equilibrium with each other.
Internal energy: It is the sum of all forms of kinetic and potential energy of the system. It is the sum of the kinetic energy of motion of the molecules and the potential energy represented by the chemical bonds between the atoms and any other intermolecular forces.
Examples:
(i) In an ideal monoatomic gas, the internal energy is in the form of translational kinetic energy of the atoms.
(ii) in a plyatomic gas, the internal energy is in the form of translational, rotational and vibrational kinetic energy of the molecules.
(iii) In a liquid or solid, the internal energy is in the form of translational, rotational and vibrational kinetic energy of the molecules and potential energy associated with the intermolecular attractive forces.
We cannot measure the absolute value of internal energy of a system. The best one can do is measure the changes in internal energy. Or we can measure it relative to some arbitrary reference state.
We use the symbol U for internal energy.