The Collaborative International Dictionary of English v.0.48:

Horror \Hor"ror\, n. [Formerly written horrour.] [L. horror, fr.
   horrere to bristle, to shiver, to tremble with cold or dread,
   to be dreadful or terrible; cf. Skr. h?sh to bristle.]
   1. A bristling up; a rising into roughness; tumultuous
      movement. [Archaic]
      [1913 Webster]

            Such fresh horror as you see driven through the
            wrinkled waves.                       --Chapman.
      [1913 Webster]

   2. A shaking, shivering, or shuddering, as in the cold fit
      which precedes a fever; in old medical writings, a chill
      of less severity than a rigor, and more marked than an
      algor.
      [1913 Webster]

   3. A painful emotion of fear, dread, and abhorrence; a
      shuddering with terror and detestation; the feeling
      inspired by something frightful and shocking.
      [1913 Webster]

            How could this, in the sight of heaven, without
            horrors of conscience be uttered?     --Milton.
      [1913 Webster]

   4. That which excites horror or dread, or is horrible; gloom;
      dreariness.
      [1913 Webster]

            Breathes a browner horror on the woods. --Pope.
      [1913 Webster]

   The horrors, delirium tremens. [Colloq.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

horror
    n 1: intense and profound fear
    2: something that inspires dislike; something horrible; "the
       painting that others found so beautiful was a horror to him"
    3: intense aversion [syn: repugnance, repulsion,
       revulsion, horror]

Moby Thesaurus II by Grady Ward, 1.0:

107 Moby Thesaurus words for "horror":
   Dracula, Frankenstein, Wolf-man, abhorrence, abject fear,
   abomination, affright, alarm, allergy, angst, animosity, animus,
   antagonism, antipasto, antipathy, anxiety, apprehension, aversion,
   awe, blue funk, bogey, bogeyman, bugaboo, bugbear, clawing,
   cold sweat, consternation, cowardice, creeping flesh, cruciation,
   crucifixion, detestation, disgust, dislike, dismay, distaste,
   distress, dread, enmity, execration, fear, fear and trembling,
   fee-faw-fum, fright, frightener, funk, ghost, ghoul, hate, hatred,
   hell, hell upon earth, hobgoblin, holocaust, holy terror,
   horrification, hostility, incubus, laceration, lancination,
   loathing, martyrdom, monster, mortal horror, nausea, nervousness,
   nightmare, odium, ogre, ogress, pain, panic, panic fear, passion,
   persecution, perturbation, phantom, phobia, purgatory, queasiness,
   rack, rancor, repugnance, repulsion, revenant, revulsion, scare,
   scarebabe, scarecrow, scarer, shock, shuddering, specter, stampede,
   succubus, terror, torment, torture, trepidation, trepidity,
   uneasiness, unholy dread, upset, vampire, werewolf, whet, wrench