Evolution window is the narrow band of the mutation step size σ, where significant progress toward the fitness/objective function's optimum was observed in evolution strategies.
There are three well-known methods to adapt the mutation step size σ in evolution strategies:
(1/5-th) Success Rule Self-Adaptation (for example through log-normal mutations) Cumulative Step Size Adaptation (CSA)
On simple functions all of them have been empirically shown to keep the step size within the evolution window.
See also Bionics Cybernetics Evolutionary Algorithm Optimization (mathematics)
References
H.-G. Beyer. Toward a Theory of Evolution Strategies: Self-Adaptation. Evolutionary Computation, 3(3), 311-347. Ingo Rechenberg: Evolutionsstrategie '94. Stuttgart: Frommann-Holzboog 1994.