Full Idea
A 'proposition' is the meaning of a sentence. More precisely, since propositions are supposed to be true or false once and for all, it is the meaning of an eternal sentence. More precisely still, it is the 'cognitive' meaning, involving truth, not poetry.
Gist of Idea
A 'proposition' is said to be the timeless cognitive part of the meaning of a sentence
Source
Willard Quine (Propositional Objects [1965], p.139)
Book Reference
Quine,Willard: 'Ontological Relativity and Other Essays' [Columbia 1969], p.139
A Reaction
Quine defines this in order to attack it. I equate a proposition with a thought, and take a sentence to be an attempt to express a proposition. I have no idea why they are supposed to be 'timeless'. Philosophers have some very odd ideas.