Extension

In any of several studies that treat the use of signs, for example in linguistics, logic, mathematics, semantics, and semiotics, the extension of a concept, idea, or sign consists of the things to which it applies, in contrast with its comprehension or intension, which consists very roughly of the ideas, properties, or corresponding signs that are implied or suggested by the concept in question.

Source
The text was copied from the Wikipedia (English) entry 'Extension' on June 3, 2009.

Link
Extension (Wikipedia)