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):

reign of terror
    n 1: any period of brutal suppression thought to resemble the
         Reign of Terror in France
    2: the historic period (1793-94) during the French Revolution
       when thousands were executed; "the Reign of the Bourbons
       ended and the Reign of Terror began"