The Collaborative International Dictionary of English v.0.48:

Terrorism \Ter"ror*ism\, n. [Cf. F. terrorisme.]
   1. The act of terrorizing, or state of being terrorized; a
      mode of government by terror or intimidation. --Jefferson.
      [1913 Webster]

   2. The practise of coercing governments to accede to
      political demands by committing violence on civilian
      targets; any similar use of violence to achieve goals.
      [PJC]

WordNet (r) 3.0 (2006):

terrorism
    n 1: the calculated use of violence (or the threat of violence)
         against civilians in order to attain goals that are
         political or religious or ideological in nature; this is
         done through intimidation or coercion or instilling fear
         [syn: terrorism, act of terrorism, terrorist act]

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "terrorism":
   Bolshevikism, Bolshevism, Carbonarism, Castroism, Jacobinism,
   Maoism, Sinn Feinism, acuteness, anarchism, animality,
   argumentum baculinum, atrocity, barbarity, big stick, bloodlust,
   brutality, despotism, destructiveness, domination, domineering,
   extremity, fascism, ferociousness, fierceness, force, furiousness,
   harshness, heavy hand, high hand, horrification, impetuosity,
   inclemency, inhumanity, intensity, iron boot, iron hand, iron heel,
   malignity, mercilessness, mindlessness, murderousness, oppression,
   pitilessness, reign of terror, revolutionism, rigor, roughness,
   sans-culotterie, sans-culottism, savagery, scaremongering,
   severity, sharpness, syndicalism, terror tactics,
   terroristic tactics, terrorization, thought control, tyranny,
   ungentleness, vandalism, vehemence, venom, viciousness, violence,
   virulence