What is Atheism?

[Nonpareil]

Yes.

It's called "understanding the null hypothesis".

That depends on what the null hypothesis actually is?

Quite right.

The null hypothesis is that gods do not exist.

The idea behind a 'null hypothesis' is that proving one explanation requires disproving the current explanation. You are proposing without justification that God's absence should be the preferred explanation that must be disproved.

No. I am stating, with perfectly adequate justification, that God's absence is the null hypothesis.

And it is. Because it is the position that posits the existence of no entities or properties of entities that is not already in evidence. It has met its burden of proof, and is the null hypothesis.

Anyways, let's see how that applies to an alternative proposition. I think it would very disingenuous for nearly any atheist to say he or she disagrees with the claim "The world is all that exists" which is functionally equivalent to lack of belief in God.

I don't see why, though your phrasing is fairly poor, so I'm not certain that your point is coming across as you intended it to.

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A deist is a theist, though. They fit the definition of theism.

Not really, for the same reason that pantheism isn't a subcategory of theism.

So... it is, then.

Because pantheism is very much a subcategory of theism.