The lack of a belief in gods.

Atheism is not a belief in itself, but an absence of a belief. Specifically, it is the absence of a belief in a god or gods. It doesn't matter the reasons for this absence. It could be based on a belief that no such god or gods could exist. Or it could be out of ignorance of the idea of a god or gods. Or it could be just someone who isn't convinced of the existence of a god, while not ruling out the possibility. Or it could be that they are convinced the basis of belief in a god is arbitrary, and believe that the arbitrary must be rejected.

Sometimes atheism is categorized as hard or soft atheism. Hard atheism is a kind of lack of belief that stems from a belief that such a god would be impossible or illogical. Soft atheism doesn't try to evaluate whether such a thing could exist or not. It just doesn't accept it as true.