A reversible process describes an ideal thermodynamic system where steps in a reaction, or changes in a physical process, can reverse direction such that the resulting entropy change for the system and surroundings is zero. If it is possible to restore a thermodynamic system and its surroundings to their original state (including the original entropic state) following a process then such a process is termed a reversible process. All other reactions and processes are irreversible.

Considering both the system and surroundings, all real processes are irreversible. Internally reversible processes demand no irreversibilities in the system and externally reversible process demand no irreversibilities outside the system.

A ideal reversible process describes a reaction or process in which the system its surroundings are in equilibrium throughout the process. In considering ideal gas expansions and contractions, for example, the system and surroundings are considered to be in equilibrium if...