An extensional adjective is an attributive adjective which can simply be interpreted as a set, because its denotation does not depend on the noun with which it is combined (e.g. 'red'). This is not true for subsective adjectives (like big in big mouse), syncategorematic adjectives (like good in good violinist) and intensional adjectives (like present in present queen).
red, female, living are extensional adjectives. The combination of an extensional adjective with a noun is interpreted as the intersection of the denotations of the adjective and the noun. Thus the expression red rose is interpreted as the intersection of the set of red things and the set of roses. (For this reason extensional adjectives are also called intersective adjectives).