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]

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.