It's not too hard to define atheism so long as you stay within the framework of theism. Atheism is the lack of belief in any number of gods greater than 0 (not the belief that there are none, but the lack of belief that there are any). Strictly speaking, you can have atheistic buddhists, atheistic religions, etc. You could also have the more common understanding of atheism, which is basically a form of skepticism - it denies the existence of things for which there is an absence of evidence (which includes deities).
There's also the lovely Klingon religious conceit that their ancestors killed all their gods because "they were more trouble than they were worth."
Don't worry about it, Major. Nobody does. It's the way they like it,