Full Idea
A meaning for a sentence is something that determines the conditions under which the sentence is true or false. It determines the truth-value of the sentence in various possible states of affairs.
Gist of Idea
Sentence meaning determines its truth-value in various situations
Source
David Lewis (General Semantics [1970], III)
A Reaction
A somewhat clearer assertion of the view originating with Frege, and championed by Davidson. Meaning is a 'something', and so opposed to the nihilistic Kripkenstein view. I agree with Lewis on that. Meanings are mental states.
Book Reference
Lewis,David: 'Philosophical Papers Vol.1' [OUP 1983], p.193