Atheism, literally, means "without a belief in god". It does not mean "The belief that god does not exist".
That means that agnostics are atheists. Since they "don't know" whether or not god exists, it means they are without a belief in god (aka; atheist).
Atheism is not a belief. It is a LACK of a belief.
I don't believe god doesn't exist...I just don't believe in god, and deny the concept of god entirely.