Full Idea
Essence is fundamentally nothing but the possibility of the thing under consideration. Something which is thought possible is expressed by a definition.
Gist of Idea
Essence is just the possibility of a thing
Source
Gottfried Leibniz (New Essays on Human Understanding [1704], 3.03)
Book Reference
Leibniz,Gottfried: 'New Essays on Human Understanding', ed/tr. Remnant/Bennett [CUP 1996], p.293
A Reaction
It is unclear whether he means 'possible modes of existence' or 'possible actions of the thing'. Leibniz sees more clearly than Aristotle that essences extend beyond the actual thing, because Leibniz is more aware of the active powers.
Related Idea
Idea 13087 The essence of a thing is its real possibilities [Leibniz, by Cover/O'Leary-Hawthorne]