The Collaborative International Dictionary of English v.0.48:

Imperfection \Im`per*fec"tion\, n. [L. imperfectio: cf. F.
   imperfection. See Imperfect, a.]
   The quality or condition of being imperfect; lack of
   perfection; incompleteness; deficiency; fault or blemish.
   [1913 Webster]

         Sent to my account
         With all my imperfections on my head.    --Shak.

   Syn: Defect; deficiency; incompleteness; fault; failing;
        weakness; frailty; foible; blemish; vice.
        [1913 Webster]

WordNet (r) 3.0 (2006):

imperfection
    n 1: the state or an instance of being imperfect [syn:
         imperfection, imperfectness] [ant: flawlessness, ne
         plus ultra, perfection]

Moby Thesaurus II by Grady Ward, 1.0:

86 Moby Thesaurus words for "imperfection":
   absence, arrear, arrearage, arrears, bad habit, baseness, beggary,
   besetting sin, blemish, bug, catch, crack, damage, decline,
   defalcation, default, defect, defection, defectiveness, deficiency,
   deficit, delinquency, demerit, deprivation, destitution, drawback,
   drought, error, failing, failure, fallibility, falling short,
   famine, fault, faute, fewness, flaw, foible, frailty, hole,
   impoverishment, inadequacy, incompetence, incompleteness,
   inferiority, infirmity, insufficiency, kink, lack, little problem,
   littleness, maladroitness, meanness, mediocrity, moral flaw, need,
   omission, peccadillo, pettiness, problem, rift, shabbiness,
   short measure, shortage, shortcoming, shortfall, sin, slump,
   smallness, snag, something missing, starvation, subnormality,
   taint, triviality, underage, unskillfulness, vice, vulgarity,
   vulnerable place, want, wantage, weak link, weak point, weak side,
   weakness