Full Idea
In possible-worlds semantics, expressions have intensions, which are functions from possible worlds to appropriate extensions (names to individuals, n-place predicates to n-tuples, and sentences to truth values, built from parts).
Clarification
'Intensions' are meanings; 'extensions' are the things referred to
Gist of Idea
The intension of an expression is a function from possible worlds to an appropriate extension
Source
John Perry (Knowledge, Possibility and Consciousness [2001], §8.1)
Book Reference
Perry,John: 'Knowledge, Possibility and Consciousness' [MIT 2001], p.170
A Reaction
Interesting. Perry distinguishes 'referential' (or 'subject matter') content, which is prior to the link to extensions - a link which creates 'reflexive' content. He is keen that they should not become confused. True knowledge is 'situated'.