Full Idea
The substance of a particular thing is variously held to be that which it was to be that thing, or the universal, or the genus, or the subject, which is that of which other entities are said, but is never itself being said-of anything else.
Gist of Idea
A substance is what-it-is-to-be, or the universal, or the genus, or the subject of saying
Source
Aristotle (Metaphysics [c.324 BCE], 1028b30)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.174
A Reaction
This formulation sounds worryingly verbal to me, but I don't suppose Aristotle meant it entirely that way.