Entropy change S is defined by the detailed imbalance of a stochastic process describing irreversible evolution. The detailed imbalance for a pair of configurations. The definition facilitates the description of entropy changes for a nonisolated system interacting with its surroundings,S decomposing intoS syst —S ext . The former, related to log(P i t /p j t ) is the change in the state entropy of the system; the latter, related to log(f ij /f ji , is the change due to surroundings. Both can be calculated for a stochastic process without recourse to thermodynamic variables. As an example,S is calculated for a nonlinear diffusion process and is shown to decrease monotonically with the time, conforming with minimum entropy production. Relationship to other definitions is discussed.