Since some atheists define atheism as simply lack of belief in God or gods, it's useful to compare that with Graham Oppy's definition:
Atheism is the rejection of theism: a-theism. Atheists maintain some or all of the following claims: that theism is false; that theism is unbelievable; that theism is rationally unacceptable; that theism is morally unacceptable. G. Oppy, "Arguments for Atheism," S. Bullivant & M. Ruse, eds. The Oxford Handbook of Atheism (OUP, 2014), 53.