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.
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            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.
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   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.
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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"