The Collaborative International Dictionary of English v.0.48:

Iniquity \In*iq"ui*ty\, n.; pl. Iniquities. [OE. iniquitee, F.
   iniquit['e], L. iniquitas, inequality, unfairness, injustice.
   See Iniquous.]
   [1913 Webster]
   1. Absence of, or deviation from, just dealing; lack of
      rectitude or uprightness; gross injustice;
      unrighteousness; wickedness; as, the iniquity of bribery;
      the iniquity of an unjust judge.
      [1913 Webster]

            Till the world from his perfection fell
            Into all filth and foul iniquity.     --Spenser.
      [1913 Webster]

   2. An iniquitous act or thing; a deed of injustice or
      unrighteousness; a sin; a crime. --Milton.
      [1913 Webster]

            Your iniquities have separated between you and your
            God.                                  --Is. lix. 2.
      [1913 Webster]

   3. A character or personification in the old English
      moralities, or moral dramas, having the name sometimes of
      one vice and sometimes of another. See Vice.
      [1913 Webster]

            Acts old Iniquity, and in the fit
            Of miming gets the opinion of a wit.  --B. Jonson.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Vice \Vice\ (v[imac]s), n. [F., from L. vitium.]
   1. A defect; a fault; an error; a blemish; an imperfection;
      as, the vices of a political constitution; the vices of a
      horse.
      [1913 Webster]

            Withouten vice of syllable or letter. --Chaucer.
      [1913 Webster]

            Mark the vice of the procedure.       --Sir W.
                                                  Hamilton.
      [1913 Webster]

   2. A moral fault or failing; especially, immoral conduct or
      habit, as in the indulgence of degrading appetites;
      customary deviation in a single respect, or in general,
      from a right standard, implying a defect of natural
      character, or the result of training and habits; a harmful
      custom; immorality; depravity; wickedness; as, a life of
      vice; the vice of intemperance.
      [1913 Webster]

            I do confess the vices of my blood.   --Shak.
      [1913 Webster]

            Ungoverned appetite . . . a brutish vice. --Milton.
      [1913 Webster]

            When vice prevails, and impious men bear sway,
            The post of honor is a private station. --Addison.
      [1913 Webster]

   3. The buffoon of the old English moralities, or moral
      dramas, having the name sometimes of one vice, sometimes
      of another, or of Vice itself; -- called also Iniquity.
      [1913 Webster]

   Note: This character was grotesquely dressed in a cap with
         ass's ears, and was armed with a dagger of lath: one of
         his chief employments was to make sport with the Devil,
         leaping on his back, and belaboring him with the dagger
         of lath till he made him roar. The Devil, however,
         always carried him off in the end. --Nares.
         [1913 Webster]

               How like you the Vice in the play?
               . . . I would not give a rush for a Vice that has
               not a wooden dagger to snap at everybody. --B.
                                                  Jonson.
         [1913 Webster]

   Syn: Crime; sin; iniquity; fault. See Crime.
        [1913 Webster]

WordNet (r) 3.0 (2006):

iniquity
    n 1: absence of moral or spiritual values; "the powers of
         darkness" [syn: iniquity, wickedness, darkness,
         dark]
    2: morally objectionable behavior [syn: evil, immorality,
       wickedness, iniquity]
    3: an unjust act [syn: injustice, unfairness, iniquity,
       shabbiness]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "iniquity":
   abomination, atrocity, bad, breach, crime, crime against humanity,
   deadly sin, delinquency, dereliction, diablerie, disgrace,
   enormity, error, evil, failure, fault, felony, genocide,
   guilty act, heavy sin, illegality, improperness, impropriety,
   indiscretion, inequitableness, inequity, inexpiable sin, infamy,
   iniquitousness, injury, injustice, knavery, lapse, malefaction,
   malfeasance, malum, minor wrong, misdeed, misdemeanor, misfeasance,
   mortal sin, nonfeasance, obliquity, offense, omission, outrage,
   peccadillo, peccancy, reprobacy, scandal, shame, sin,
   sin of commission, sin of omission, sinful act, slip, tort,
   transgression, trespass, trip, undueness, unjustness, unlawfulness,
   unmeetness, unutterable sin, venial sin, villainy, wrong,
   wrongdoing, wrongfulness, wrongness



Bouvier's Law Dictionary, Revised 6th Ed (1856):

INIQUITY. Vice; contrary to equity; injustice.
     2. Where, in a doubtful matter, the judge is required to pronounce, it
is his duty to decide in such a manner as is the least against equity.