The Collaborative International Dictionary of English v.0.48:

Injustice \In*jus"tice\, n. [F. injustice, L. injustitia. See
   In- not, and Justice, and cf. Unjust.]
   1. Lack of justice and equity; violation of the rights of
      another or others; iniquity; wrong; unfairness;
      imposition.
      [1913 Webster]

            If this people [the Athenians] resembled Nero in
            their extravagance, much more did they resemble and
            even exceed him in cruelty and injustice. --Burke.
      [1913 Webster]

   2. An unjust act or deed; a sin; a crime; a wrong.
      [1913 Webster]

            Cunning men can be guilty of a thousand injustices
            without being discovered, or at least without being
            punished.                             --Swift.
      [1913 Webster]

WordNet (r) 3.0 (2006):

injustice
    n 1: an unjust act [syn: injustice, unfairness, iniquity,
         shabbiness]
    2: the practice of being unjust or unfair [syn: injustice,
       unjustness] [ant: justice, justness]

Moby Thesaurus II by Grady Ward, 1.0:

124 Moby Thesaurus words for "injustice":
   abstractionism, asymmetry, atrocity, bias, bigotry, breach,
   coloring, contrariety, crime, crime against humanity, damage,
   deadly sin, deformation, delinquency, dereliction, difference,
   discrimination, disequilibrium, disparity, disproportion,
   disservice, distortion, enormity, error, evil, exaggeration,
   expressionism, failure, false coloring, falsification, fault,
   favoritism, felony, garbling, genocide, great wrong, grievance,
   gross injustice, guilty act, harm, heavy sin, heterogeneity, hurt,
   hyperbole, imbalance, imposition, impropriety, inaccuracy,
   inadequacy, indiscretion, inequality, inequitableness, inequity,
   inexpiable sin, infraction, infringement, iniquity, injury,
   insufficiency, irregularity, lapse, litotes, malefaction,
   malfeasance, malpractice, malum, minor wrong,
   miscarriage of justice, mischief, miscoloring, misdeed,
   misdemeanor, misdrawing, misfeasance, mispainting, misquotation,
   misreport, misrepresentation, misstatement, misteaching,
   mortal sin, nonfeasance, nonrealism, nonuniformity, odds, offense,
   omission, one-sidedness, outrage, overbalance, overdrawing,
   overstatement, partiality, partisanship, peccadillo, peccancy,
   perversion, prejudice, raw deal, ruin, shortcoming, sin,
   sin of commission, sin of omission, sinful act, slanting, slip,
   tort, transgression, trespass, trip, twisting, unbalance,
   understatement, unevenness, unfair discrimination, unfairness,
   unjustness, unutterable sin, venial sin, villainy, violation,
   wrong, wrongdoing



The Devil's Dictionary (1881-1906):

INJUSTICE, n.  A burden which of all those that we load upon others
and carry ourselves is lightest in the hands and heaviest upon the
back.