A defective verb (/dəfektɪv( v)ɜ:b/) is a verb with incomplete conjugation, or one which cannot be used in some other way as normal verbs can, i.e. defectivity. Defective verbs cannot be conjugated in certain tenses, aspects, or moods.
There are sundry other defective verbs in English, particularly beware.
Impersonal verbs are in a sense semi-defective.