Source : Webster's Revised Unabridged Dictionary (1913)

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.

            Terror seized the rebel host.         --Milton.

   2. That which excites dread; a cause of extreme fear.

            Those enormous terrors of the Nile.   --Prior.

            Rulers are not a terror to good works. --Rom. xiii.
                                                  3.

            There is no terror, Cassius, in your threats.
                                                  --Shak.

   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.

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

   {Reign of Terror}. (F. Hist.) See in Dictionary of Noted
      Names in Fiction.

   Syn: Alarm; fright; consternation; dread; dismay. See
        {Alarm}.

Source : WordNet®

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"