One who has disbelief in any form of a higher being. It is not necessary for an atheist to disprove the existance of a higher being, because those who are not atheists, cannot prove that a higher being exists. Both atheism and religion are based on faith.
Atheist: I don't believe in God.
Non-Atheist: Prove that God doesn't exist!
Atheist: ¡Prove that he/she does exist!