differentia

noun

1. A distinguishing feature which marks a species off from other members of the same genus.

   “That character, it seems to me, should be regarded as the practically important differentia of religion for our purpose; and just what it is can easily be brought out by comparing the mind of an abstractly conceived Christian with that of a moralist similarly conceived.”

   “If genera are different and co-ordinate, their differentiae are themselves different in kind. Take as an instance the genus ‘animal’ and the genus ‘knowledge’. ‘With feet’, ‘two-footed’, ‘winged’, ‘aquatic’, are differentiae of ‘animal’; the species of knowledge are not distinguished by the same differentiae.”

   “In the case of a word like “surfboard,” the differentiae seem pretty clear. How is this board different from all other boards?”