(WIP)
The structural complexity of a system is the size of the set of minimal causal components embedded in a system. The minimal causal architecture of a system is referred to as an epsilon-machine representation of that system. The field of computational mechanics can be thought of as a theory of structural complexity.
Structural complexity is a complementary metric to the 'randomness' inherent to a system. It is highest at some point between purely random and purely ordered.