One who does not believe in a god or gods.
Sometimes stereotyped to be intolerant to religons.
They are not all intolerant. It is simply not believing in a higher being. If one should be free to be a certain religon without being intolerant, one should be free to not believe in a god(s) without being intolerant.
*Not to be confused with agnostic*
How I would hope it would go-
Christain: I'm Catholic.
Athiest: Oh, that's fine.
Christian: Well, what are you?
Athiest: Oh, I'm an athiest.
Christian: Oh, well that's cool.
Ways it may go but not many people would not want-
Religous person: I'm Catholic
Athiest: That's cool.
RP: Well, what about you?
Athiest: Well, I'm an athiest.
RP: You son of a bitch! You're going to Hell, stupid!
RP: I'm Catholic
Athiest: Oh my god! You're so stupid thee is no higher being! The Bible is fictional.
Athiest: No! Shut-up!