The Collaborative International Dictionary of English v.0.48:

Animosity \An`i*mos"i*ty\, n.; pl. Animosities. [F.
   animosit['e], fr. L. animositas. See Animose, Animate, v.
   t.]
   1. Mere spiritedness or courage. [Obs.] --Skelton.
      [1913 Webster]

            Such as give some proof of animosity, audacity, and
            execution, those she [the crocodile] loveth.
                                                  --Holland.
      [1913 Webster]

   2. Violent hatred leading to active opposition; active
      enmity; energetic dislike. --Macaulay.
      [1913 Webster]

   Syn: Enmity; hatred; opposition. -- Animosity, Enmity.
        Enmity be dormant or concealed; animosity is active
        enmity, inflamed by collision and mutual injury between
        opposing parties. The animosities which were continually
        springing up among the clans in Scotland kept that
        kingdom in a state of turmoil and bloodshed for
        successive ages. The animosities which have been
        engendered among Christian sects have always been the
        reproach of the church.
        [1913 Webster]

              Such [writings] as naturally conduce to inflame
              hatreds and make enmities irreconcilable.
                                                  --Spectator.
        [1913 Webster]

              [These] factions . . . never suspended their
              animosities till they ruined that unhappy
              government.                         --Hume.
        [1913 Webster]

WordNet (r) 3.0 (2006):

animosity
    n 1: a feeling of ill will arousing active hostility [syn:
         animosity, animus, bad blood]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "animosity":
   acerbity, acid, acidity, acidulousness, acrimony, animus,
   antagonism, antipathy, asperity, bad blood, bad feeling, bile,
   bitter feeling, bitter resentment, bitterness,
   bitterness of spirit, causticity, choler, contempt, detestation,
   disaffinity, enmity, feud, gall, gnashing of teeth, hard feelings,
   hardheartedness, hatred, heartburning, hostility, ill blood,
   ill feeling, ill will, immediate dislike, loathing, malevolence,
   malice, personality conflict, rancor, rankling, resentment,
   slow burn, soreness, sourness, spleen, vendetta, venom, virulence,
   vitriol