A subspecies is a race within a species that shows identifiable characteristics different from other subspecies. It is usually geographically separate from other subspecies.

A subspecies is identified by the third part of the scientific name, e.g. in the case of Great Horned Owl
Bubo virginianus virginianus is the subspecies found in eastern North America while
Bubo virginianus nacurutu is found in South America.

Because the North American race, Bubo virginianus virginianus, has its subspecies name the same as the species name it is known as the "nominate" subspecies. The nominate subspecies is the first of the genus to be discovered and described.