noun: a lack of belief in the existence of God or gods
noun: the doctrine or belief that there is no God

Hughes
"Science and Revelation are mutually, though gradually, clearing each other; but as a little warmth of the rising sun only calls up the very mists which are to be dissipated by its more powerful shining, so a vague and chilling popular unbelief is to be dispelled, not by withholding knowledge, but by shedding abroad all possible light. Christianity has one most dangerous mental foe, and that is ignorance." Ignorance is the parent of Atheism; but we should carefully distinguish between "sinful doubt" and candid inquiry, the former of which generally arises from a too great love of, and belief in, our own mental powers. [Harmonies of Tones and Colours, Reflections on the Scheme3, page 45]

"The owlet Atheism,
Sailing on obscure wings across the noon,
Drops his blue fringed lids and shuts them close,
And, hooting at the glorious sun in heaven,
Cries out, Where is it?" [Coleridge]. [Harmonies of Tones and Colours, Reflections on the Scheme4, page 46]

See Also
atheistic materialism