Full Idea
Intensions are functions that assign a set to the expression at each possible world, ..so the semantic value of 'red' is the function that maps each possible world to the set of things in that world that are red.
Gist of Idea
Intensions are functions which map possible worlds to sets of things denoted by an expression
Source
Chris Swoyer (Properties [2000], 4.2)
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.23
A Reaction
I am suddenly deeply alienated from this mathematical logicians' way of talking about what 'red' means! We need more psychology, not less. We call things red if we imagine them as looking red. Is imagination a taboo in analytical philosophy?