adjective

1. not finished or complete; lacking in something
2. not perfect; having a defect, fault, or error
3. Gram. in certain inflected languages, designating or of the tense of a verb that indicates a past action or state as uncompleted, continuous, customary, or going on at the same time as another: “was writing” and “used to write” are English forms corresponding to the imperfect tense in such languages
4. Music designating an interval of a major or minor third or sixth

Etymology: ME inperfit < OFr imparfit < L imperfectus: see in- & perfect

noun

1. the imperfect tense
2. a verb in this tense