Full Idea
Matter ex hypothesi is what ultimately underlies (to huperkeimenon) a thing; it is that from which something comes to be and which remains as a non-coincidental component in the thing's make-up.
Gist of Idea
Matter underlies things, composes things, and brings them to be
Source
David Wiggins (Substance [1995], 192a30)
Book Reference
Aristotle: 'Physics', ed/tr. Waterfield,Robin [OUP 1996], p.31
A Reaction
This is an interesting prelude to the much more comprehensive discussion of matter in Metaphysics, where he crucially adds the notion of 'form', and gives it priority over the underlying matter.