Reversible and irreversible processes:
Thermodynamic system which is capable of restoring the original state of it by reversing the factors responsible for occurrence of process is called as reversible system and the thermodynamic process involved is called as reversible process.
Hence upon reversal of the process there shall be no trace of process being occurred, that is state changes during forward direction of occurrence of process are exactly similar to the states passed through by the system during reversed direction of process
It is obvious that such reversibility can be realized only if the system maintains the thermodynamic equilibrium of it throughout the occurrence of process.
Irreversible systems are those that do not maintain equilibrium during occurrence of process. Various factors responsible for the non attainment of equilibrium are the reasons responsible for irreversibility Presence of friction, etc