René Descartes defines extension as the property of existing in more than one dimension. Or in layman's terms, the property of "taking up space". For Descartes, the primary characteristic of matter is extension, just as the primary characteristic of mind is consciousness
Another common philosophical definition of extension is that 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'. It is widely argued that the meaning of a word is sometimes just the things that the word picks out, or applies to.
For a different meaning of extension, see Field extension.