Full Idea
An item is abstract if 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 come before the mind by concentrating on a part of what is presented
Source
Keith Campbell (The Metaphysic of Abstract Particulars [1981], §1)
Book Reference
'Properties', ed/tr. Mellor,D.H. /Oliver,A [OUP 1997], p.126
A Reaction
I think this point is incredibly important. Pure Fregean semantics tries to leave out the psychological component, and yet all the problems in semantics concern various sorts of abstraction. Imagination is the focus of the whole operation.