Full Idea
A 'substance' might be defined to be an object which does not depend for its existence upon any other object (where dependency is defined in terms of necessity.
Gist of Idea
A 'substance' is an object which doesn't depend for existence on other objects
Source
E.J. Lowe (The Possibility of Metaphysics [1998], 1.3)
Book Reference
Lowe,E.J.: 'The Possibility of Metaphysics' [OUP 2001], p.10
A Reaction
I'm inclined to leave out 'substance', which has too much historical baggage, and talk of minimal things having 'identity', and proper things having 'essence'.