What is Imperfect?

Legal Definition
The imperfect (abbreviated IMPERF) is a verb form, found in various languages, which combines past tense (reference to a past time) and imperfective aspect (reference to a continuing or repeated event or state). It can therefore have meanings similar to the English "was walking" or "used to walk." It contrasts with preterite forms, which refer to a single completed event in the past.

Traditionally, the imperfect of languages such as Latin and French is referred to as one of the tenses, although in fact it encodes aspectual information in addition to tense (time reference). It may be more precisely called past imperfective.

When used in relation to English, "imperfect" refers to forms much more commonly called past progressive or past continuous (like was doing or were doing). These are combinations of past tense with specifically continuous or progressive aspect.

The term can take on specific conventional meanings in the grammars of particular languages. In German, Imperfekt was used to refer to the simply conjugated past tense (to contrast with the Perfekt or compound past form), but the term Präteritum (preterite) is now preferred, since the form does not carry any implication of imperfective aspect.

"Imperfect" comes from the Latin imperfectus "unfinished", because the imperfect expresses an ongoing, uncompleted action. The equivalent Ancient Greek term was paratatikós "prolonged".
-- Wikipedia
Legal Definition
That which is incomplete.

2. This term is applied to rights and obligations. A man has a right to be relieved by his fellow-creatures, when in distress; but this right he cannot enforce by law; hence it is called an imperfect right. On the other hand, we are bound to be grateful for favors received, but we cannot be compelled to perform such imperfect obligations. Vide Poth. Ob. arc. Pr«liminaire; Vattel, Dr. des Gens, Prel. notes, 17; and Obligations.
-- Bouviers Law Dictionary
Legal Definition
As used in various legal compound terms, this word means defective or incomplete; wanting in some legal or formal requisite; wanting in legal sanction or effectiveness; as in speaking of imperfect "obligations," "ownership," "rights," "title," "usufruct," or "wav." See those nouns.
-- Black's Law Dictionary