There’s a discussion at Talking Philosophy of how to define atheism. It’s basically about the difference between saying atheism is not believing that there is a god and saying that it is belief there there is no god. Me, I would define it the first way first and then add the second as a more affirmative or energetic version – but what I wouldn’t do is leave out the first. I think the first is 1) an important part of atheism and 2) a version of atheism that is more useful to a lot of people than the more affirmative version is. It has to be possible to be definitely non-theist without having to be affirmative about it.
It does seem fair to say that atheism doesn’t (or shouldn’t) really apply to people who’ve never thought about the matter at all – atheism does seem to be more affirmative than that. So the definition should include that. I suggested ‘Atheism is, at a minimum, explicit nonbelief in a god.’ ‘Explicit’ means that the question has been considered, and that belief has been at the very least declined, and perhaps refused or rejected. But that still doesn’t entail affirmative belief that there is no god – but it also doesn’t entail the ‘oh gee I just don’t know, I have no idea’ popularly attributed to agnosticism these days. It’s just a No. No means No.