The Collaborative International Dictionary of English v.0.48:

Imperfect \Im*per"fect\, a. [L. imperfectus: pref. im- not +
   perfectus perfect: cf. F imparfait, whence OE. imparfit. See
   Perfect.]
   1. Not perfect; not complete in all its parts; wanting a
      part; deective; deficient.
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            Something he left imperfect in the state. --Shak.
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            Why, then, your other senses grow imperfect. --Shak.
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   2. Wanting in some elementary organ that is essential to
      successful or normal activity.
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            He . . . stammered like a child, or an amazed,
            imperfect person.                     --Jer. Taylor.
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   3. Not fulfilling its design; not realizing an ideal; not
      conformed to a standard or rule; not satisfying the taste
      or conscience; esthetically or morally defective.
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            Nothing imperfect or deficient left
            Of all that he created.               --Milton.
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            Then say not man's imperfect, Heaven in fault;
            Say rather, man's as perfect as he ought. --Pope.
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   Imperfect arch, an arch of less than a semicircle; a skew
      arch.

   Imperfect cadence (Mus.), one not ending with the tonic,
      but with the dominant or some other chord; one not giving
      complete rest; a half close.

   Imperfect consonances (Mus.), chords like the third and
      sixth, whose ratios are less simple than those of the
      fifth and forth.

   Imperfect flower (Bot.), a flower wanting either stamens or
      pistils. --Gray.

   Imperfect interval (Mus.), one a semitone less than
      perfect; as, an imperfect fifth.

   Imperfect number (Math.), a number either greater or less
      than the sum of its several divisors; in the former case,
      it is called also a defective number; in the latter, an
      abundant number.

   Imperfect obligations (Law), obligations as of charity or
      gratitude, which cannot be enforced by law.

   Imperfect power (Math.), a number which can not be produced
      by taking any whole number or vulgar fraction, as a
      factor, the number of times indicated by the power; thus,
      9 is a perfect square, but an imperfect cube.

   Imperfect tense (Gram.), a tense expressing past time and
      incomplete action.
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WordNet (r) 3.0 (2006):

imperfect tense
    n 1: a tense of verbs used in describing action that is on-going
         [syn: progressive, progressive tense, imperfect,
         imperfect tense, continuous tense]