Formal systems involving intensional notions, especially modality.
British Dictionary definitions for intensional
(logic) (of a predicate) incapable of explanation solely in terms of the set of objects to which it is applicable; requiring explanation in terms of meaning or understanding Compare extensionalSee also opaque context, Electra paradox
the set of characteristics or properties by which the referent or referents of a given word are determined: thus, the intension of marsupial is the set containing the characteristics suckling its young and having a pouchCompare extension (sense 11a)
philosophy A description of properties, e.g. intensional equality, that relate to how an object is implemented as opposed to extensional properties which concern only how its output depends on its input. (1995-01-12)