This is something I have had so many discussions about ever since I became an active atheist, and it puzzles me how so many people don't understand what it means to be an atheist. There are so many wrong - or at least not meaningful - definitions of atheism that I feel it is important to properly define exactly what you mean when you call yourself an atheist. To me, anyone who is not a theist is by definition an atheist. There is a single claim: A god or gods exist. If you are convinced that that claim is true, you are a theist. If you are not convinced that this claim is most likely true, you are an atheist.
Now, granted, atheism can be further subdivided into varying degrees, but as far as I am concerned they are all still atheism, merely subsets of atheism. A weak atheist, also colloquially known as an 'agnostic', although this is technically incorrect as agnosticism deals with knowledge not belief, is someone who is neither convinced that gods do exist or that gods do not exist. A strong atheist, also sometimes known as an 'anti-theist', is someone who is convinced that gods do not exist. So by my definition, and in my experience the most common definition and the one which I personally find most useful, there is no middle ground between theism and atheism and logically one cannot exist as that would be a violation of the Law of Excluded Middle.