Atheism is simply the state of nonbelief in a deity.
Atheism is a spectrum too. At one end you have small-a atheists who would believe in a deity if presented with compelling proof. At the other end you have people with a fervent belief in no deities who would not be persuaded by evidence.
I don't like to see science presented as a belief system. Science is a tool for establishing fact about the natural world in a methodical and reproducible way. For debating the existence of gods, you need philosophy.