Full Idea
Since being as belonging to a category expresses the 'isness' of things, and belongs to all ten Aristotelian categories, essence must be something all the natures that sort different beings into genera and species have in common.
Gist of Idea
Essence is something in common between the natures which sort things into categories
Source
Thomas Aquinas (De Ente et Essentia (Being and Essence) [1267], p.92)
Book Reference
Aquinas,Thomas: 'Selected Philosophical Writings', ed/tr. McDermott,Timothy [OUP 1993], p.92
A Reaction
I like this because it is the essence which does the sorting, not the sorting which defines the essence (which seems to me to be a deep and widespread confusion).