The Collaborative International Dictionary of English v.0.48:

Atrocity \A*troc"i*ty\, n.; pl. Atrocities. [F. atrocit['e],
   L. atrocitas, fr. atrox, atrocis, cruel.]
   1. Enormous wickedness; extreme heinousness or cruelty.
      [1913 Webster]

   2. An atrocious or extremely cruel deed.
      [1913 Webster]

            The atrocities which attend a victory. --Macaulay.
      [1913 Webster]

WordNet (r) 3.0 (2006):

atrocity
    n 1: the quality of being shockingly cruel and inhumane [syn:
         atrocity, atrociousness, barbarity, barbarousness,
         heinousness]
    2: an act of atrocious cruelty [syn: atrocity, inhumanity]

Moby Thesaurus II by Grady Ward, 1.0:

174 Moby Thesaurus words for "atrocity":
   abomination, abuse, acuteness, affront, animality, aspersion,
   atrociousness, awfulness, bad, bane, banefulness, barbarity,
   befoulment, blight, bloodlust, breach, brickbat, brutality,
   contempt, contumely, corruption, crime, crime against humanity,
   cruelty, crying evil, cut, damage, deadly sin, defilement,
   delinquency, dereliction, desecration, despite, despoliation,
   destruction, destructiveness, detriment, direness, disgrace,
   disservice, dreadfulness, dump, enormity, error, evil, extremity,
   failure, fault, felony, ferociousness, fierceness, flagitiousness,
   flout, flouting, force, furiousness, genocide, gibe, great wrong,
   grievance, grimness, gross injustice, guilty act, harm, harshness,
   havoc, heavy sin, heinousness, hideousness, horribleness,
   horridness, horror, humiliation, hurt, ignominy, ill,
   ill-treatment, ill-usage, ill-use, impetuosity, imposition,
   impropriety, inclemency, indignity, indiscretion, inexpiable sin,
   infamy, infection, inhumanity, iniquity, injury, injustice, insult,
   intensity, jeer, jeering, knavery, lapse, malefaction, malfeasance,
   malignity, maltreatment, malum, mercilessness, mindlessness,
   minor wrong, miscarriage of justice, mischief, misdeed,
   misdemeanor, misfeasance, mistreatment, mock, mockery, molestation,
   monstrousness, mortal sin, murderousness, nonfeasance, obliquity,
   offense, omission, outrage, peccadillo, peccancy, pitilessness,
   pity, poison, pollution, profanation, put-down, raw deal,
   reprobacy, rigor, roughness, sacrilege, savagery, scandal, scoff,
   scurrility, severity, shame, sharpness, sin, sin of commission,
   sin of omission, sinful act, slip, taunt, terrible thing,
   terribleness, terrorism, the worst, tort, toxin, transgression,
   trespass, trip, uncomplimentary remark, ungentleness,
   unutterable sin, vandalism, vehemence, venial sin, venom, vexation,
   viciousness, villainy, violation, violence, virulence, wickedness,
   woe, wrong