  We can similarly speak of the content of a singular term with respect to a context, and the extension or referent of that term, with respect to a context and a world. For example, the definite description ‘the person who invented bifocals’ has the same content with respect to all contexts. (Its content is some type of structured entity that has various properties and relations as constituents, such as the property of being a person, the relation of inventing, and so on.) This content determines different individuals at different worlds, because different people invent bifocals at different worlds. So, the referent of ‘the person who invented bifocals’, with respect to a context and a world, varies from world to world, because the person who invented bifocals varies from world to world. (We assume here, with Kaplan, that definite descriptions are singular terms rather than quantifier phrases.)