(A group of), often geographically isolated, population(s) which is morphologically distinct from other populations of the same species. The term subspecies replaces the older term 'race', although the latter is still used in the ornithological literature. Subspecies are designated trinominally (three terms), a third term being added to the specific binomen of the species. The "original" species is called the 'nominate subspecies' and is indicated by the third term being a repetition of the second. A subspecies is often difficult to define, as it is usually based on samples from a population continuum.