If a process can be carried out in such a way that the effects produced by it on the system as well as the surroundings can be completely restored to their initial state and no changes are left in any of the systems taking part in the process or to the surroundings, then the process is said to be a reversible process. In this process, the system passes through a succession of equilibrium state or states that differ from equilibrium only infinitesimally. Such a process must occur slowly in a controlled manner. If a process does not satisfy these conditions, it is said to be irreversible process, i.e., the processes which produce a permanent change in the thermodynamic state of the system and cannot be retraced in the opposite manner, are known as irreversible process.

Examples of reversible processes are an infinitesimally slow expansion and compression of ideal gas at constant pressure, all the mechanical processes taking place under the action of conservative force, etc.

Decay matter, flowing of current through a conductor, rusting of iron, etc. is examples of irreversible processes.