Full Idea
The older sense of 'abstract' applies to universals, where a universal like 'redness' is abstracted from red particulars; it is the one associated with the many. In mathematics, the notion of 'group' or 'vector space' perhaps fits this pattern.
Gist of Idea
The older sense of 'abstract' is where 'redness' or 'group' is abstracted from particulars
Source
James Robert Brown (Philosophy of Mathematics [1999], Ch. 2)
Book Reference
Brown,James Robert: 'Philosophy of Mathematics' [Routledge 2002], p.12
A Reaction
I am currently investigating whether this 'older' concept is in fact dead. It seems to me that it is needed, as part of cognitive science, and as the crucial link between a materialist metaphysic and the world of ideas.