Intension is the physical meaning of a form. It is the complement of extension, where extension is the conceptual range to which the form applies.

Every concept can be defined and understood in two complementary ways. We may think of it as the name of a class of objects or subsidiary concepts; or we may think of what it means. We define a concept in extension when we specify all the elements of the class it refers to. And we define a concept in intension when we try to explain its meaning analytically in terms of other concepts at the same level.

Christopher Alexander, Notes on the Synthesis of Form