Extension

The extension of a word, phrase, or concept is the set of things it extends to, or applies to. So the extension of the word 'dog' includes all the dogs in the world: Fido, Rover, Lassie, Rex, and so on. The extension of the phrase 'Wikipedia readers' includes each person who has ever read Wikipedia, including the person reading these words right now.

'Extension' is one of the meanings of 'meaning'. The meaning of a word is, sometimes, just the things that the word picks out, or applies to.