We have this debate over how to define
atheism periodically, and the popular choice among atheists on the internet is usually that it should be defined as "lack of belief in gods". And then people end up going around in circles over the confusion over the difference between definitions and meanings.
My preferred definitions of theism and atheism are:
1) theism = acceptance of a belief in one or more gods
2) atheism = rejection of a belief in one or more gods
That is, I see both as positive beliefs about something, and I don't count people who lack an understanding of what gods are as either theists or atheists. I believe that this accurately captures the way people use words like
theism and
atheism. We do not think of babies or animals as "atheists", and those who argue that they ought to be considered atheists are engaging in a
converse accident fallacy. That is, they are using a general definition to override exceptions to standard usage of the word
atheism. They argue that, because of the way they have chosen to define the word, babies and animals must then be considered atheists, since beings that are unable to have a belief in gods are ipso facto atheists.
A complication in these arguments is that definitions are precise heuristic statements, whereas meanings are very complex bundles of associated concepts. All words exhibit a certain amount of ambiguity (different precise word senses) and vagueness (lack of precision). So, to the extent that the concept of "god" shifts around, the concept of "atheism" shifts around.