My definition comes from commonly available works of scholarships on atheism, by atheists, etc. Take a trip to
www.atheism.org (though it is a redirect to the more provocatively named
www.infidels.org) find info on their goal "to defend a nontheistic worldview which holds that the natural world is all that there is, a closed system in no need of supernatural explanation and sufficient unto itself". I don't offer this as an explanation of atheism's existence, but more a very easily obtainable example of prevalent trends in atheist thought. To be frank, I decided to look around and 'www.atheism.org' was the first thing that I thought of as a good starting point.
Looking up through the comments, sixate, an outspoken atheist in many other posts on this boards, seems to concur. I would say that your definition errs on the side of simplicity and bows to space constraints in normal published media. Most online dictionaries are simply electronic versions of actual print dictionaries and thus must keep definitions terse.
I would hope that you do not render a fairly popular and well thought out system of thought such as atheism down to one line. I would certainly feel remiss to describe Christianity, Liberalism, or any other religion or belief/non-belief structure in such an oversimplified way.
Are you simply rebutting to rebutt, or are you an atheist that believes in spooks and the fae?