Definition of "differentia"

That character, it seems to me, should be regarded as the practically important differentia of religion for our purpose.

1902, William James, The Varieties of Religious Experience, Folio Society, published 2008, page 38

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.

1928, E. M. Edghill, Categories, translation of original by Aristotle

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

2017, Kory Stamper, Word By Word, Vintage, published 2018, page 116