The definition of atheism

There are two problems with the definition of atheism. The first problem is that there are no "atheist police" who go around making sure that everyone who self-identifies as atheist does so for the "right" reasons. So you have people who have studied a lot of philosophy, apologetics and theology coexisting with people who have simply rejected the religion they've grown up with, and might therefore have a narrow view of what constitutes religion (and a lot of others in between). Second, you do have philosophically educated atheists, who have the related problem of how to succinctly express non-compliance with theism, a family of related beliefs, beliefs we did not create nor organize into a family. Neither atheism nor theism are beliefs: they are families of beliefs.

Thus atheists typically declare the most general definition as lack of belief that any God exists. This sounds like a cop-out — many atheists have very definite beliefs about God — but it's not: we are not characterizing any specific beliefs, but rather what almost all self-identified atheists have in common: we do not subscribe to any of the family of beliefs that do in fact positively declare the existence of God.

Within the family of beliefs about God, we can identify several individual categories or sub-families. While individual atheists have different opinions, and some might dissent from the specific conclusions, there are areas of broad agreement about our attitude towards these categories:

• God is the sort of being who exists, and whose existence can be proven (e.g. McDowell): We have examined the evidence, and the evidence not only fails to show that such a God exists, but also shows that no such God exists. (Alternatively, "God" denotes some prosaic concept like "love" or "beauty", which does exist but in the typical atheist's opinion does not really deserve the word "God".)
• God is the sort of being who exists, but whose existence can neither be proven nor disproven (e.g. Catholic theology): First, such believers are often disingenuous; they want to prove God's existence, but when presented with the overwhelming disconfirmatory evidence, they retreat into "God can neither be proven nor disproven." More importantly, it is a category error to even talk about the existence of something whose existence can neither be proven nor disproven: "Whereof one cannot speak," quoth Wittgenstein, "thereof one must be silent."
• The word "God" does not denote a concept to which one can associate the concept of "existence" (e.g. Spong): First, are you sure you're not already an atheist? Second, are you really saying anything at all?

It's not our fault that theologians and apologists have made a semantic hash out of the word "God". Fundamentally, the atheist position is that all this God talk is simply nonsense, even as we recognize that the nonsense has some variety and cleverness in its construction.