Atheist, n.
Etymology: [Gr. athéiste.]
1. One who disbelieves or denies the existence of a God, or supreme intelligent Being.
2. A godless person. [Obs.]
Syn. -- Infidel; unbeliever.
Note: See Infidel.
Shades of meaning
Fernald has no entry of its own for atheist; he lists it under skeptic.
Where he writes about it:
The atheist denies that there is a God; the deist admits the existence of God, but denies that the Christian Scriptures are a revelation from him; the agnostic denies either that we do know or that we can know whether there is a God. — from skeptic