Full Idea
If something is 'abstract' it is got before the mind by an act of abstraction, that is, by concentrating attention on some (but not all) of what is presented.
Gist of Idea
Abstractions are formed by the mind when it concentrates on some, but not all, the features of a thing
Source
J.P. Moreland (Universals [2001], Ch.3)
Book Reference
Moreland,J.P.: 'Universals' [Acumen 2001], p.53
A Reaction
Presumably it usually involves picking out the behavioural or causal features, and leaving out the physical features - though I suppose it works for physical properties too…