Given a word class C whose members inflect for morphological categories M1 ... Mn, where each Mi takes different values V1 ... Vn; then the inflection of a subclass S of C is defective iff it lacks at least one Mi or one Vi. Derivatively, the members of S are called defective.

Example: The German substantival pronouns man and einer, both ‘one’ (the latter being partly suppletive of the former), lack a genitive.

Lacking a value V means that V cannot be expressed on the members of S. This must be distinguished from the case that V1 has no distinct expression, but rather the same expression as V2 of the same paradigm. This is syncretism, not defectivity.