Full Idea
Essence may be taken for the very being of any thing, whereby it is, what it is. And thus the real internal, but generally in substances, unknown constitution of things, whereon their discoverable qualities depend, may be called their essence.
Gist of Idea
Essence is the very being of any thing, whereby it is what it is
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 3.03.15)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.417
A Reaction
Fine cites this as following the Aristotelian definitional account of essence, rather than the account in terms of necessities. Locke goes on to distinguish 'real' from 'nominal' essence.