differentia
Concise Oxford English Dictionary © 2008 Oxford University Press:
differentia /ˌdɪfəˈrɛnʃɪə/
▶noun (pl. differentiae /-ʃɪiː/) a distinguishing mark or characteristic.
■ chiefly Philosophy an attribute that distinguishes a species of thing from other species of the same genus.
'differentia' also found in these Oxford entries:

