The Collaborative International Dictionary of English v.0.48:

Terror \Ter"ror\, n. [L. terror, akin to terrere to frighten,
   for tersere; akin to Gr. ? to flee away, dread, Skr. tras to
   tremble, to be afraid, Russ. triasti to shake: cf. F.
   terreur. Cf. Deter.]
   1. Extreme fear; fear that agitates body and mind; violent
      dread; fright.
      [1913 Webster]

            Terror seized the rebel host.         --Milton.
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   2. That which excites dread; a cause of extreme fear.
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            Those enormous terrors of the Nile.   --Prior.
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            Rulers are not a terror to good works. --Rom. xiii.
                                                  3.
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            There is no terror, Cassius, in your threats.
                                                  --Shak.
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   Note: Terror is used in the formation of compounds which are
         generally self-explaining: as, terror-fraught,
         terror-giving, terror-smitten, terror-stricken,
         terror-struck, and the like.
         [1913 Webster]

   King of terrors, death. --Job xviii. 14.

   Reign of Terror. (French Hist.) See in Dictionary of Noted
      Names in Fiction.
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   Syn: Alarm; fright; consternation; dread; dismay. See
        Alarm.
        [1913 Webster]

WordNet (r) 3.0 (2006):

terror
    n 1: an overwhelming feeling of fear and anxiety [syn: panic,
         terror, affright]
    2: a person who inspires fear or dread; "he was the terror of
       the neighborhood" [syn: terror, scourge, threat]
    3: a very troublesome child [syn: terror, brat, little
       terror, holy terror]
    4: the use of extreme fear in order to coerce people (especially
       for political reasons); "he used terror to make them confess"

Moby Thesaurus II by Grady Ward, 1.0:

116 Moby Thesaurus words for "terror":
   Dracula, Frankenstein, Mafioso, Wolf-man, Young Turk, abject fear,
   affright, alarm, anxiety, awe, beast, beldam, berserk, berserker,
   blue funk, bogey, bogeyman, bomber, bruiser, brute, bugaboo,
   bugbear, consternation, cowardice, demon, devil, dismay, dragon,
   dread, fear, fearfulness, fee-faw-fum, fiend, fire-eater,
   firebrand, fright, frightener, funk, fury, ghost, ghoul, goon,
   gorilla, gun, gunsel, hardnose, hatchet man, hell-raiser, hellcat,
   hellhound, hellion, hobgoblin, holy terror, hood, hoodlum,
   hooligan, horrification, horror, hothead, hotspur, incendiary,
   incubus, intimidation, killer, mad dog, madcap, monster, mug,
   mugger, muscle man, nightmare, ogre, ogress, panic, panic fear,
   phantom, phobia, plug-ugly, rapist, revenant, revolutionary,
   rodman, roughneck, savage, scare, scarebabe, scarecrow, scarer,
   scourge, she-wolf, shock, specter, spitfire, stampede,
   strong-arm man, succubus, termagant, terrorist, tiger, tigress,
   torpedo, tough, tough guy, trepidation, trepidity, trigger man,
   ugly customer, unholy dread, vampire, violent, virago, vixen,
   werewolf, wild beast, witch, wolf