The Krohn-Rhodes complexity, also called the group complexity or simply "the complexity," of a finite semigroup 
is the smallest number of groups in a wreath product of finite
groups and finite aperiodic semigroups
of which 
is a divisor.
Krohn-Rhodes Complexity
See also
Krohn-Rhodes Theorem, Krohn-Rhodes TheoryExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Krohn-Rhodes Complexity." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Krohn-RhodesComplexity.html