They have the belief that there are no deities.
Not that they don't believe anything at all.
an acceptance that something exists or is true, especially one without proof
I think this definition applies more to the "god is real" context rather than "god is not real" context. Also emphasis on the "without proof" part. Many Atheists will argue that they have proof that most religions are bogus, such as doing silly experiments like asking questions such as "Why doesn't he cure cancer/aids/diseases?" or "Why doesn't he show himself?"
pretty much nothing, you could easily argue that anything is a religion
Everything is a religion? Not possible. I don't worship a god(s) or have a belief on anything like whether god exists or doesn't, it does not cross my mind, therefore I am not religious.