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