Reversible and irreversible processes

The course of a thermodynamic process can be either reversible or irreversible (reversible × irreversible).

If an equilibrium event takes place in a given system in one direction and then in the opposite direction and the system returns to its original state without changes occurring in the surrounding bodies, We call such a process a reversible event. Actual events only approximate the ideal return, and only if they proceed slowly enough.

All real events are irreversible. In one direction, they proceed spontaneously without external influence, and in order for the action to proceed in the opposite direction, it is necessary to supply energy from an external source.

Source

 * KUBATOVA, Senta. Biofot [online]. [cit. 2011-01-31]. .