Source : Webster's Revised Unabridged Dictionary (1913)

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

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

   3. (Politics) The theories and practices of the Nihilists.

Source : WordNet®

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}]
     3: complete denial of all established authority and
        institutions