I really think that if people want a close definition of atheism, they need to investigate the nature of 'a belief.'
Atheists don't believe in God? OK, but putting aside for the moment what 'God' might mean... what does it mean 'to believe' something, or 'to have a belief'?
Let's talk about plate tectonics, forty years ago. Did geologists believe in plate tectonics? Well, I'd say that most of them kinda believed and kinda didn't -- depending on how you phrased the question, whether they'd studied the latest evidence, how distracted they were about personal problems, and the politics of their academic departments.
I guess what bothers me most is that some people seem to see belief and non-belief as ON/OFF things. In my view, that's just the language fooling them. Since we have opposite words, we think that there are actually opposite things (belief states) out there to which the words refer. Actually I think that the existence of the words themselves may tend to push some folks into one camp or the other. They assume that they must either believe or not believe.
That's kinda sorta how it seems to me, anyway.
I see belief as a continuum, myself.