A metaphysical or epistemological position (depending on how one understands scientific explanations) that an object can be defined in terms of its role in a system, instead of, say, the stuff of which it is made. A mousetrap is defined functionally because we understand it in terms of what it does (namely, cpature mice). A mousetrap could be made out of just about anything -- wood, wire, plastic, poison, etc. -- but as long as it returns a captured mouse for a free one, then it is a mousetrap.
The view that structural properties, and not the physical nature of something, is its defining characteristic. Two physically different systems can have the same kinds of experiences.