Actually, lack of belief would be agnosticism, since a belief hasn't been asserted. In atheism, there is something being asserted, that is, that there is no God. You quite clearly assert this, and rather than being a lack of belief in God, it is a belief in the lack of God. As an atheist, it isn't that you don't have a belief concerning God, which would be a lack of belief, it is that you believe there actually is no God. You believe it, really and truly. If you did not, then you wouldn't be atheist. If you didn't believe it, you wouldn't assert it.
IF you were to say, I'm atheist because there is no evidence or proof for a God, but if there were, then I guess I'd believe in it, then you're not actually atheist, you're agnostic. You really haven't decided, you're sitting on the fence until there's proof one way or the other. The atheist positively asserts that there is no God, that there is no evidence or proof for a God, nor will there ever be evidence or proof found. It isn't a matter of, "I'll believe it when I see it." It's a matter of, "I'll never believe it, because I'll never see it."
Wherever there is a positive assertion, there is a belief. Unless I'm mistaken, the atheist makes the positive assertion "there is no God."