The imperfect (abbreviated IMPERF) is a verb form that combines past tense (reference to a past time) and imperfective aspect (reference to a continuing or repeated event or state). It can 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 it actually encodes aspectual information in addition to tense (time reference). It may be more precisely called past imperfective.
English has no general imperfective and expresses it in different ways. The term "imperfect" in English refers to forms much more commonly called past progressive or past continuous (e.g. "was doing" or "were doing"). These are combinations of past tense with specifically continuous or progressive aspect. In German, Imperfekt formerly referred 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".