Reversible process
An idealised process that is quasi-static and frictionless, so it can be run backwards through the same states — the limiting ideal behind the second law.
Definition
A reversible process is one that can be reversed by an infinitesimal change in conditions, retracing the very same sequence of equilibrium states in the opposite direction and leaving no net change in either the system or its surroundings. It must be quasi-static (slow enough that the system is in equilibrium throughout) and free of dissipative effects such as friction, turbulence, or unrestrained expansion.
No real process is exactly reversible — every actual change generates some entropy and loses some capacity to do work. Reversibility is a limiting ideal, approached only as a process is made infinitely slow and frictionless. Its importance is that a reversible process delivers the maximum possible work between two states, setting the benchmark against which real processes fall short.
The second law of thermodynamics is framed in terms of reversible processes: the Carnot engine, which runs reversibly, achieves the highest efficiency any engine can have between two reservoirs, and entropy change is defined through the reversible heat ∫δQ_rev/T. The gap between reversible and irreversible work is the seed of irreversibility itself.
History
Conceived by Sadi Carnot in 1824 as the reversible ideal engine, and made the formal cornerstone of the second law by Clausius and Kelvin in the 1850s, who tied reversibility to the existence of entropy.