(religion) A person who does not believe in deities or gods, ranging from those who are certain of their non-existence to those who simply doubt their existence.

(uncommon) A person who does not believe in a particular deity (but may believe in another deity).

(sometimes proscribed) A person who does not believe in any religion (not even a religion without gods); a nonreligious person. (Can we verify(+) this sense?)

(transitive, very rare) To make someone an atheist.

Of or relating to atheists or atheism; atheistic.