In short, if you are asked, "Do you believe in one or more gods?", and your answer is, "No", you're an atheist.

Labeling controversies aside, that is technically all there is to it.

The public conception of atheism at large is that atheists assert, absolutely, that there is no god, and thus have faith. This is a massive misconception, as the majority of self-described atheists don't make this sort of absolute claim.