A universal separation index, the extent of separation, is proposed and mathematically described. This index is easily calculated, normalized, invariant to a permutation in indices, and conceptually similar to the De Donder extent of reaction. It applies to any type of separation system, any initial or final level of component purity, any concentration profile (if one exists), any initial amount of components, multicomponent systems, and both separation and mixing processes. The index is employed to characterize one of the simplest of all separation systems—the single equilibrium stage.