The term subspecies refers to a biologically distinct subdivision of a biological species. The Y'bith, for example, are a subspecies of the Bith species. While obviously similar to other Bith, they differed in their weaker eyesight, more sensitive skin, thicker bones, stronger hands and feet, and ductile mouth structures. A related term was race. While members of different races within a species are capable of interbreeding and producing viable offspring, members of different species generally could not. In casual discourse, however, the term "race" would often be used where species would be technically correct (especially when referring to different sentient species.) In addition, it was often difficult to tell if a population was a race, a subspecies, or a distinct species, and the division was often arbitrary or cultural. The Theelin, for example, were capable of interbreeding with baseline Humans, but were referred both as a race and as a distinct species.