Symmachus wrote:
That is true to an extent, but beyond mere clinical definition the word has a social life of its own, like it or not. For example, take Mak's comment
I favor defining atheism as how self-described atheists and atheist philosophers prefer to use it with a little deference given to etymology. Happily, that consistently is something to the effect of "lack of belief in gods." I can only think of one major atheist philosopher, William Rowe, who defends the definition of atheism in terms of belief that God does not exist. (Agnosticism, by contrast, is usually defined by atheists as a position on the possibility of knowledge of God, which corresponds with its historical use.)
I try to emphasize the notion of rejection rather than mere unbelief because I think atheism is an attitude about the idea of a god. This requires one to have heard of the concept of "god" and positively not accept it. I do not, for example, think newborn infants are atheists just because I think they probably do not have belief in deities at that point. When I make this point to fellow atheists, they always agree in my experience. Atheism isn't just nontheism. It's a stance on the
idea of theism.
There is a popular idea about atheists, heavily reinforced by polemics against atheists, that contends atheism is belief that God does not exist. Often I see it described as unassaiable certainty that God does not exist. But I don't see why they should get to define the term. This is usually a rhetorical strategy that describes the atheist position as one they think is easier to refute or portray as unreasonable. Agnosticism is usually defined by the very same people as a wishy-washy uncertainty. Atheists who think they have compelling reasons to reject theistic justification are just defined right off the map.