I can't say that I am *the* authority on the definition of the word "atheist", but in my experience the way it has been used for the most part is as an expression of the belief that there is no god. That is in agreement with the way it is formally defined by some sources, for example:
"1.the doctrine or belief that there is no God.
2.disbelief in the existence of a supreme being or beings."
Note that "disbelief" is simply believing that something is not. That is quite different than the absence of belief. There are scarce few westerners who actively practice the elimination of belief. Atheism is just another one of the beliefs floating around. However, I think it's a stretch to call it a religion.