rom The Collaborative International Dictionary of English v.0.48 [gcide]:

  Nihilist \Ni"hil*ist\, n. [Cf. F. nihiliste. See {Nihilism}.]
     1. One who advocates the doctrine of nihilism; one who
        believes or teaches that nothing can be known, or asserted
        to exist.
        [1913 Webster]
  
     2. (Politics) A member of a secret association (esp. in
        Russia), which is devoted to the destruction of the
        present political, religious, and social institutions.
        [1913 Webster]

From WordNet (r) 2.0 [wn]:

  nihilist
       n 1: someone who rejects all theories of morality or religious
            belief
       2: an advocate of anarchism [syn: {anarchist}, {syndicalist}]


From The Collaborative International Dictionary of English v.0.48 [gcide]:

  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]

From WordNet (r) 2.0 [wn]:

  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