Full Idea
There are three conceptions of abstractness: 1) non-spatial entities, the opposite of 'concrete' (e.g. numbers and universals); 2) an entity logically incapable of a separate existence (e.g. an apple's colour); 3) Fregean abstractions from concepts.
Gist of Idea
Abstractions are non-spatial, or dependent, or derived from concepts
Source
E.J. Lowe (The Possibility of Metaphysics [1998], 10.1)
Book Reference
Lowe,E.J.: 'The Possibility of Metaphysics' [OUP 2001], p.211
A Reaction
[Lowe p.218 explains the third one] Lowe rejects the third one, and it is a moot point whether the second one could actually be classed as an entity (do they have identity-conditions?), so the big issue is the first one.