I'd love to see more dictionaries have a better definition of atheism however I disagree with your definition. Atheism is merely lacking belief in god or gods. There are plenty of atheists on this planet that believe in other supernatural things.
I always explain it as Atheism = Everything that isn't Theism. It includes Agnostics, Apatheists, strong Atheists, weak Atheists, "not-sure" and everything in between.
What's happening is that the word is growing beyond its basic definition into a colloquial understanding, and it doesn't help that some prominent atheists are stoking this fire. I think Dawkins does a lot of harm when he says things like 'atheists are kind, good people who simply reject supernatural explanations.' That's true in the sense that all of the people he describes are atheists, but not all atheists fit that description. It's not a technically correct definition. When Dawkins speaks of atheists like this, he's actually talking about skeptic secular humanists. There are some who might argue that he's trying to attach a positive image to atheists themselves, but the problem is that once we start touting atheism as something more than the rejection of theism, we have all these bad atheists we have to explain, and I don't think the argument that "well Stalin wasn't doing it in the name of atheism" is really convincing to believers.
Dawkins is unwittingly trying to turn atheism into an ideology, a code of behavior, a label. He's presenting it as something MORE than just the rejection of theism, and I personally think that does more harm than good. The real problem with Stalin, Pol Pot, et. al. is that they didn't follow a rational philosophy, and I think that needs to be pointed out. Theism is only one kind of irrationality that harms human societies, but it is all irrationality in general that needs to addressed.
I went round and round on the local city forum with a theist about this, trying to explain that my lack of belief in something did not necessarily mean that I believe in something else. There is a large overlap of skepticism / rationalism / atheism but one doesn't require the other. Just because I don't believe something is black doesn't mean I think it is white.
This is the real point. There appears (even among my more educated and reasonable friends and associates) a tendency to concoct some 'filler' for god. Frankly, I imagined there had to be one when I still clung to the vague 'spiritual' idea of god. Honestly, discovering that faith actually served no required purpose whatsoever was the most enlightening and uplifting moment in my life. Really, I honestly could not believe how much fear was based on my sense of need for faith. It was astounding.