The truth is that atheism is the absence of the belief that is theism. Theism is the belief that at least one god of some sort exists; a-theism is the absence of a belief that some sort of god exists. In standard English the prefix "a" is defined as "without, absence, privation, lacking." Thus atheism, or a-theism, has to be defined defined as without, the absence of, the privation of, or lacking a belief in at least one god of some sort.
Of course this doesn't mean that atheists have no beliefs whatsoever; on the contrary like any human being of course atheists have beliefs. Some of those beliefs are also related to atheism, theism, religion, and gods. Just because a person doesn't believe in the existence of any gods doesn't mean that they don't have any beliefs whatsoever related to the subject.
Put most simply, the absence of belief in gods is no more a belief itself than the absence of political beliefs (a-political) is itself a political position or the absence of synchronicity (a-synchronous) is itself a way of being synchronous.