The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

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

The Devil's Dictionary (1881-1906):

NIHILIST, n.  A Russian who denies the existence of anything but
Tolstoi.  The leader of the school is Tolstoi.