It depends which dictionary you read. The one that says atheism is the disbelief in God has made atheism redundant with agnosticism. The one that says that atheism is the denial of God has allowed both terms to have unique meanings. If two terms mean the same thing then one is unnecessary. That is not to say that humans don't have terms that mean the same thing, it just means that it makes issues more confusing to do so. This is not a technical argument it is a logical efficiency claim.Why do you think that terms such as these should not have overlapping meanings? Agnostics are atheists, by definition, because all that is required to be classified as an atheist is a "lack of belief in the existence of God", which agnostics surely do. There are many terms in the english language with overlapping meanings, and there is no requirement that atheism and agnosticism be mutually exclusive.
Deism is not interchangeable with theism. Deism denies a personal God and theism mandates one that is personal. They cannot be used interchangeably.Agnostics make up a subcategory of Atheism just as Deists make up a subcategory of Theism. Agnosticism really does not battle atheism in any way due to the fact that agnosticism is based on a lack of or impossibility of knowledge, whereas atheism is based on a lack of belief.
Maybe I should have stated this as the way things should be, not what is true of the mess man has made of them.
But atheism is not a mere lack of belief in something. If atheism is just the lack of god-belief, then tables and chairs are atheists. For they lack god-belief. Am I being uncharitable? Suppose someone defines atheism more carefully as lack of god-belief in beings capable of having beliefs. That is still unacceptable. Consider a child who lacks both god-belief and god-disbelief. If lacking god-belief makes him an atheist, then lacking god-disbelief makes him a theist. So he is both, which is absurd.
Obviously, atheism is not a mere lack of belief, but a definite belief, namely, the belief that the world is godless.
Maverick Philosopher: A Bad Reason for Thinking that Atheism is not a Religion