The problem with common language is that words can take on a range of meanings, and so may mean different things to different people.
I consider myself an atheist. When I use the word atheist, I mean that I believe that no gods exist - a bit stronger that simply saying that I don't believe in God (which can imply a bit of uncertainty). I am fairly certain that there is no god. In fact, there are religions that do not include the belief in a deity, so to be precise when I use the word atheist I mean that I believe that the material world is all that exists.
Of course, other people use the word to mean something slightly different. This does tend, sometimes, to cause a bit of confusion in conversations. That's why I sometimes get a little pedantic about asking for precise definitions of words. (That, and my mathematical instincts.)
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