Full Idea
The proposition expressed by a sentence can be thought of as a set of possible worlds, the worlds for which its intension delivers truth.
Clarification
'Intension' is (roughly) meaning
Gist of Idea
A proposition is a set of possible worlds for which its intension delivers truth
Source
John Perry (Knowledge, Possibility and Consciousness [2001], §8.1)
Book Reference
Perry,John: 'Knowledge, Possibility and Consciousness' [MIT 2001], p.170
A Reaction
It has always struck me as important to hang on to the concept of a 'proposition' (over and above sentences). This idea gives a metaphysics for the concept, and the 'language of thought' offers appropriate brain structures. A neat picture.