The Collaborative International Dictionary of English v.0.48:

Nihilism \Ni"hil*ism\, n. [L. nihil nothing: cf. F. nihilisme.
   See Annihilate.]
   1. Nothingness; nihility.
      [1913 Webster]

   2. The doctrine that nothing can be known; scepticism as to
      all knowledge and all reality.
      [1913 Webster]

   3. (Politics) The theories and practices of the Nihilists.
      [1913 Webster]

WordNet (r) 3.0 (2006):

nihilism
    n 1: a revolutionary doctrine that advocates destruction of the
         social system for its own sake
    2: the delusion that things (or everything, including the self)
       do not exist; a sense that everything is unreal [syn:
       nihilistic delusion, nihilism]
    3: complete denial of all established authority and institutions

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "nihilism":
   Jacobinism, New Left, Old Left, agnosia, anarchism,
   anarcho-syndicalism, anarchy, antinomianism, block, blocking,
   chaos, confusion, criminal syndicalism, cynicism, defeatism,
   delirium, delusion, delusion of persecution, dismalness, disorder,
   disorderliness, disorganization, disorientation, disruption,
   extreme left, extreme left wing, extremism, flight of ideas,
   gloominess, gloomy outlook, hallucination, hallucinosis,
   left-wing extremism, lynch law, malism, mental block,
   mental confusion, misrule, mob law, mob rule, mobocracy,
   negativism, nihilistic delusion, ochlocracy, paralogia, pessimism,
   primal chaos, psychological block, radicalism, radicalization,
   rebellion, retreatism, revolution, revolutionism, sans-culotterie,
   sans-culottism, syndicalism, tohubohu, turmoil, ultraconservatism,
   ultraism, uncheerfulness, unruliness