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]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "nihilist":
   Bolshevik, Cassandra, Eeyore, Jacobin, Wobbly, anarch, anarchist,
   anarcho-syndicalist, animal, annihilator, anthropophagite,
   antinomian, arsonist, barbarian, beast, biblioclast, bomber, brute,
   burner, calamity howler, cannibal, cynic, defeatist, demolisher,
   destroyer, dynamitard, dynamiter, exterminator,
   extreme left-winger, extremist, hun, hyena, iconoclast,
   idol breaker, idoloclast, killjoy, left-wing extremist,
   lunatic fringe, malist, man-eater, mild radical, mutineer,
   negativist, parlor Bolshevik, parlor pink, pessimist, pink, pinko,
   prophet of doom, radical, rebel, red, retreatist, revolutionary,
   revolutionist, ruiner, sans-culotte, savage, seek-sorrow, shark,
   subversive, syndicalist, terrorist, tiger, ultra, ultraist, vandal,
   wild man, worrywart, wrecker, yippie



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.