Full Idea
A particular is something (not necessarily an object) which instantiates but is not itself instantiated. Universals, on the other hand, necessarily have instances (or, at least, are instantiable).
Gist of Idea
Particulars are instantiations, and universals are instantiables
Source
E.J. Lowe (The Possibility of Metaphysics [1998], 10.4)
Book Reference
Lowe,E.J.: 'The Possibility of Metaphysics' [OUP 2001], p.217
A Reaction
This is Lowe's proposal for distinction. It at least establishes the direction of dependency, but I find the notion of 'instantiation' to be as obscure and problematic as the Platonic notion of 'partaking' (see in Ontology|Universals|Platonic Forms).