atheist (n.)

1570s, "godless person, one who denies the existence of a supreme, intelligent being to whom moral obligation is due," from French athéiste (16c.), from Greek atheos "without god, denying the gods; abandoned of the gods; godless, ungodly," from a- "without" (see a- (3)) + theos "a god" (from PIE root *dhes-, forming words for religious concepts).

The existence of a world without God seems to me less absurd than the presence of a God, existing in all his perfection, creating an imperfect man in order to make him run the risk of Hell. [Armand Salacrou, "Certitudes et incertitudes," 1943]

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Definitions of atheist from WordNet
atheist (n.)
someone who does not believe in the existence of a god;
atheist (adj.)
related to or characterized by or given to atheism;
atheist leanings
Synonyms: atheistic / atheistical