The Collaborative International Dictionary of English v.0.48:

Atheism \A"the*ism\, n. [Cf. F. ath['e]isme. See Atheist.]
   1. The disbelief or denial of the existence of a God, or
      supreme intelligent Being.
      [1913 Webster]

            Atheism is a ferocious system, that leaves nothing
            above us to excite awe, nor around us to awaken
            tenderness.                           --R. Hall.
      [1913 Webster]

            Atheism and pantheism are often wrongly confounded.
                                                  --Shipley.
      [1913 Webster]

   2. Godlessness.
      [1913 Webster]

WordNet (r) 3.0 (2006):

atheism
    n 1: the doctrine or belief that there is no God [syn:
         atheism, godlessness] [ant: theism]
    2: a lack of belief in the existence of God or gods

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "atheism":
   agnosticism, apostasy, backsliding, denial, desertion, disbelief,
   discredit, faithlessness, fall from grace, gentilism, heresy,
   impiety, impiousness, inability to believe, incredulity,
   infidelity, irreligion, irreverence, lapse, lapse from grace,
   minimifidianism, misbelief, nonbelief, nullifidianism, recidivism,
   recreancy, rejection, secularism, unbelief, unbelievingness,
   undutifulness