Definition of "imperfective"

In the last chapter, we observed the rise of an imperfective marker in English, the Progressive, as the dominant and surviving member of a wider paradigm of imperfective aspectual functions in Old English, but which were no longer marked in the same way in later stages of the history of English.

2006, Debra Ziegeler, Interfaces with English Aspect: Diachronic and Empirical Studies, John Benjamins Publishing Company, page 89

The former is the basic imperfective subjunctive, while the latter is prominent or intensified. […] While the present subjunctive is imperfective in aspect, the perfect subjunctive is imperfective in aspect and spatially proximate.

2008, Constantine R. Campbell, Verbal Aspect and Non-indicative Verbs: Further Soundings in the Greek of the New Testament, Peter Lang, page 65

Consider, then, the result of applying these tense morphemes to a perfective or an imperfective verb.

2012, Ronald W. Langacker, Essentials of Cognitive Grammar, Oxford University Press, page 157

Examples of past imperfectives are shown in (42).

2005, Jane H. Hill, A Grammar of Cupeño, University of California Press, page 132

These putative indeterminate imperfectives are innovative in form precisely because they add a manner meaning to the range of lexical meanings available to Proto-Slavic motion verbs.

2010, Viktoria Hasko, Renee Perelmutter, “Introduction: Verbs of motion in Slavic languages: Paths for exploration”, in Viktoria Hasko, Renee Perelmutter, editors, New Approaches to Slavic Verbs of Motion, John Benjamins Publishing Company, page 5

Past imperfectives are also unproblematic because imperfectives are mass-like in nature, hence contractible.

2012, Ronald W. Langacker, Essentials of Cognitive Grammar, Oxford University Press, page 157