An Etymological Dictionary of Astronomy and Astrophysics
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A physical process in which the combined → entropy of the → system and the → environment increases. During an irreversible process the system is not in equilibrium at all instances of time. Most of the processes in nature are irreversible. → reversible process.

→ irreversible; → process.

Any physical process which can be performed in the reverse direction, the whole series of changes constituting the process being exactly reversed. → irreversible process.

→ reversible; → process.