[HPLovecraft]

Before you can define atheism and lump it in with religion, you need to define religion itself, since atheism is merely a response to it.

Daniel C. Dennett's definition of religion, which I tend to ascribe to, for simplicity's sake:

A social system whose participants avow belief in a supernatural agent or agents whose approval is to be sought.

He goes on to say that this is a starting place, and not carved in stone, and that any "religion" that goes beyond this has evolved beyond the point of being called a religion. Buddhism, for instance, would not fall into the category of religion. Not because Buddhists don't believe in a God (many do), but because a supernatural agent whose approval is to be sought is irrelevant in the context of the teachings of Buddhism itself.