Intension and Extension describe two ways of indicating the meaning of a word or name.

Intension assumes the word has an intrinsic meaning, perhaps simply by definition and thus "analytic."

Extension is the set of objects in the world to which the word corresponds. There is a special kind of definition called "ostensive" which defines a word by pointing to those objects. Because extension involves things in the world it is called "synthetic."

The mathematician Gottlob Frege distinguished intension and extension by the German words Sinn and Bedeutung (which translate usually as Sense and Meaning.

Vienna Circle philosophers, notably Rudolf Carnap described intension and extension as semantisch and pragmatisch (semantic and pragmatic).

Willard van Orman Quine used the terms Meaning and Reference for intension and extension.

Note the rough correlation between a priori and a posteriori and the ideas of intension and extension.

Thus intension/extension can be seen to be part of the "great dualism."