Atheism is not believing in god. It is the ABSENCE of belief in god. If you have never heard of god before then you are an Atheist. Babies for example are Atheists. Animals are Atheists.
See, that's precisely why I disagree with the "lack of belief" definition of atheism. It just makes things silly. (And, not to mention, makes assumptions that you cannot possibly know are true. Maybe all babies are born singing god's name, as Sinead O'Conner asserts.)
I believe that atheism is an opinion. It is the belief that gods don't exist, or the disbelief that gods exist. It's a position.
I also am strongly suspicious of the claim of mere "lack of belief" coming from someone who has thought about this subject enough to form an opinion about it.
The way I see it, the only people who can truly claim to have a mere "lack of belief" are:
1. People who have never heard of god concepts, and therefore have no knowledge about the subject.
2. People who are completely apathetic about the subject and have never thought about it long enough to form an opinion. (note: if you are debating on a religious forum about it, this means you do not qualify for this category.)
3. "True" agnostics, in the popular sense. These are the people who believe that the evidence for and against god is equally weighted. They believe that it is just as likely for god to exist as it is for god not to exist, and therefore, cannot be said to have an opinion either way.