Full Idea
A structure is the abstract form of a system, focussing on the interrelationships among the objects, and ignoring any features of them that do not affect how they relate to other objects in the system.
Gist of Idea
A structure is an abstraction, focussing on relationships, and ignoring other features
Source
Stewart Shapiro (Structure and Ontology [1989], 146), quoted by James Robert Brown - Philosophy of Mathematics Ch.4
Book Reference
Brown,James Robert: 'Philosophy of Mathematics' [Routledge 2002], p.60
A Reaction
I find this account very attractive, even though it appeals to supposedly outmoded psychological abstractionism. It seems pretty close to Aristotle's view of things. Shapiro's account must face up to Frege's worries about these matters.