Thermodynamic system that is capable of restoring its original state by reversing the factors responsible for occurrence of the process is called reversible system and the thermodynamic process involved is called reversible process.
Thus upon reversal of a process there shall be no trace of the process being occurred, i.e., state changes during the forward direction of occurrence of a process are exactly similar to the states passed through by the system during the reversed direction of the process.
It is quite obvious that such reversibility can be realised only if the system maintains its thermodynamic equilibrium throughout the occurrence of process.
Irreversible systems are those, which do not maintain equilibrium during the occurrence of a process. Various factors responsible for the non–attainment of equilibrium are generally the reasons responsible for irreversibility Presence of friction, dissipative effects etc.
