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]