A narrow geographical line that separates the natural ranges of two distinct animal populations. The best-characterized house mouse hybrid zone occurs in Central Europe and separates M. m. domesticus to the west and M. m. musculus to the east.

Hybrid zones are locations where the hybrid offspring of two divergent populations (sometimes defined as subspecies or races) are prevalent and form a cline (Barton & Hewitt, 1985). Precise definitions of hybrid zones vary, some insist on increased variability of fitness within the zone, others that hybrids be identifiably different from parental forms and others that they represent secondary contact alone (Murray, 1985). They occur at the area of contact between two closely related but genetically different populations, each regarded as parental forms.