Location in which species' ranges overlap and mating between these different species therefore can occur.

It is at hybrid zones that species boundaries are tested, that is, where hybridization occurs and therefore postzygotic barriers become apparent and prezygotic barriers are selected. It is also at hybrid zones that introgression occurs, or at least is initiated.

With allopatric speciation, when two population's ranges again come to overlap, that overlap need not be across the entire ranges of both populations but instead my be limited to subsets of one or both ranges, i.e., limited to hybrid zones.