The range of definitions for atheism used today:
A: The belief that God (or gods) do not exist.
A': The lack of belief in the existence of God (or gods).
I find most atheists to purport A’. They have their reasons but I find it to be the case as to shift the burden of proof to the theist. I don’t want to generalize this to all atheists but many will hold to A’ in definition but dialogue as if A is true. I dialogued with a few atheists from the Lynchburg Area Secular Humanists about two years ago who conceded this point once I caught them trying to do this.
There’s an obvious logical distinction. I believe atheism asserts that it is a belief that there is no God (A), though, historically it has been referred to as a lack of belief (A’). To say it is lack of belief, I believe, is to commit the fallacy of semantic obsolescence (assigning a meaning that the word in question used to have in earlier times; but that is no longer found within the live, semantic range of the word).
If one holds to a “lack of belief in God” they have an existential quantifying statement (there is such a God that I don’t believe in). If the atheist claims “I believe there is no God” (A), they apply a universal quantifier. I find A’ to be more consistent with an agnostic (mostly soft agnostics) and not an atheist. If the atheist holds to A’ then God could really exist and he just not believe in him in the atheist’s own paradigm. That’s fine, it’s just whether or not he is consistent in his claims and his arguments. I think a lot of time is wasted on squabbling over this definition when engaging in dialogue because once you start talking you’ll know where the atheist really stands anyways. It’s not a hill to die on, the dialogue is more important.