Not a religion, but it is a belief system. I would probably call it more of a life choice depending how you came into atheism.
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[quote]it is a belief system.[/quote]no it isn't. there are no beliefs necessarily associated with atheism; atheism is the rejection of the belief that God exists, nothing more. some atheists will hold the belief that God doesn't exist, but that is not a necessary condition of being an atheist.