Full Idea
Semantic values in extensional semantics are extensions, like individuals for terms, and sets for predicates. In modal semantics we have intensions, functions from worlds to appropriate extensions.
Gist of Idea
Extensional semantics has individuals and sets; modal semantics has intensions, functions of world to extension
Source
Robert C. Stalnaker (Counterparts and Identity [1987], 2)
Book Reference
Stalnaker,Robert C.: 'Ways a World Might Be' [OUP 2003], p.113
A Reaction
It seems obvious that the meaning of a word like 'giraffe' must include possible giraffes, as well as actual and deceased giraffes.