Full Idea
The what-it-was-to-be-that-thing [to ti en einai, essence] is, for each thing, what it is taken to be [kath' hauto, in virtue of itself] per se.
Gist of Idea
A thing's essence is its intrinsic nature
Source
Aristotle (Metaphysics [c.324 BCE], 1029b13)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.178
A Reaction
[Translations is brackets from Vasilis Politis] Aristotle's other definition of essence is in terms of definition - Idea 10963 and Idea 11292.
Related Ideas
Idea 10963 A thing's essence is what is mentioned in its definition [Aristotle, by Lawson-Tancred]
Idea 11292 Things have an essence if their explanation is a definition [Aristotle]