Full Idea
For Aristotle, the essence of a thing is what is stated in the definition of that thing.
Gist of Idea
Essence is what is stated in the definition
Source
report of Aristotle (Metaphysics [c.324 BCE], defs) by Vassilis Politis - Aristotle and the Metaphysics 7.5
Book Reference
Politis,Vasilis: 'Aristotle and the Metaphysics' [Routledge 2004], p.207