A person who does not believe in any religion (not even a religion without gods)
A person who does not believe in a particular deity (or any deity in a particular pantheon), notwithstanding that they may believe in another deity.
A person who has no belief in any deities, such as a person who has no concept of deities.
A person who does not believe in deities.
A person who believes that no deities exist (especially, one who has no other religious belief).
A person who rejects belief that any deities exist (whether or not that person believes that deities do not exist).
To make someone an atheist.
Of or relating to atheists or atheism; atheistic.
A non-Muslim, especially a Christian, an infidel; especially as used by Turkish people with particular reference to Christians such as Greeks, Armenians, Bulgarians, Serbs and Assyrians.