The relation dS = δQrev/T defines entropy change for what kind of process?

The relation dS = δQrev/T defines entropy change for a reversible process.

Entropy is a state function, so its change depends only on the initial and final equilibrium states. Heat, by contrast, is path-dependent; the subscript “rev” indicates that the heat must be evaluated along a reversible path connecting those states. For a finite change, this becomes ΔS = ∫δQrev/T.

A reversible process is an idealized, infinitely slow process with no friction, turbulence, finite temperature gradients, or other dissipative effects. In an irreversible process, the equality does not generally hold for the actual heat transfer; entropy production makes the system’s entropy change greater than the corresponding heat-transfer term. Isochoric and isobaric describe constant volume and constant pressure, not reversibility: either can occur reversibly or irreversibly.

The distinction became central in nineteenth-century thermodynamics. Rudolf Clausius introduced entropy in 1865 while formalizing the limits on heat engines. The SI unit of entropy is joule per kelvin, and entropy is commonly represented by S.