The Collaborative International Dictionary of English v.0.48:

Atheist \A"the*ist\, n. [Gr. ? without god; 'a priv. + ? god:
   cf. F. ath['e]iste.]
   1. One who disbelieves or denies the existence of a God, or
      supreme intelligent Being.
      [1913 Webster]

   2. A godless person. [Obs.]
      [1913 Webster]

   Syn: Infidel; unbeliever.
        [1913 Webster]

   Note: See Infidel.
         [1913 Webster] Atheistic

WordNet (r) 3.0 (2006):

atheist
    adj 1: related to or characterized by or given to atheism;
           "atheist leanings" [syn: atheist, atheistic,
           atheistical]
    n 1: someone who denies the existence of god

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "atheist":
   Sabbath-breaker, apostate, backslider, blasphemer, deserter,
   disbeliever, gentile, heathen, infidel, minimifidian, nonbeliever,
   nullifidian, pagan, recidivist, recreant, renegade, sacrilegist,
   secularist, unbeliever