In thermodynamics, irreversible means that that there was a transformation content increase in the system in one heat cycle. Irreversible processes and reactions are quantified by the Clausius inequality. All natural processes are irreversible.

The following are related quotes:

“The process of heat conduction cannot be completely reversed by any means. This expresses the same idea as the wording of Clausius, but without requiring an additional clarifying explanation. A process which in no manner can be completely reversed I called a ‘natural’ one. The term for it in universal use today, is: ‘irreversible’.”
Max Planck (c.1940), Scientific Autobiography

Reversible processes are not, in fact, processes at all, they are sequences of states of equilibrium. The processes which we encounter in real life are always irreversible.”
Arnold Sommerfeld (c.1850) [1]

See also
● Irreversible process
● Irreversible reaction
Irreversible thermodynamics

1. Hokikian, Jack. (2002). The Science of Disorder: Understanding the Complexity, Uncertainty, and Pollution in Our World (pg. 25). Los Feliz Publishing.

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