There is more than one form of atheism. The most common distinction between kinds of atheists is that between weak atheists and strong atheists. However, the only one thing common to all kinds of atheists is a lack of belief in deities. Therefore, that's all you can say atheism is: a lack of belief.
There are two different issues to consider when labelling someone in reference to this topic. These issues are belief and knowledge. They are not the same thing. Belief is usually based off of knowledge, but it doesn't have to be. Contrarily, lack of belief is usually based off of lack of knowledge, but it doesn't have to be.
Taking both belief and knowledge into account, there are four possible positions on this topic. One is belief in a deity without knowledge of the deity's existence. Two is belief in a deity while claiming knowledge of the deity's existence. Three is a lack of belief without knowledge of the deity's nonexistence. Four is a lack of belief while claiming knowledge of the deity's nonexistence. How shall we label them? We already have a term that references knowledge (or a lack thereof): agnosticism. The term, coined by Thomas Henry Huxley, was a response to the "Gnostics" that lived centuries earlier. These Gnostics claimed to have secret knowledge concerning Christianity and God, and hence drew their name from the Greek word for knowledge, gnosis. Mr. Huxley wasn't so arrogant and decided to label himself as possessing no such knowledge of the divine. He used the Greek prefix meaning "without/lacking" (a-) and appended it to the word gnostic. So we have "agnostic" which means "without knowledge," and in this context it means "without knowledge of the divine/deities."
So, going back to our four, the two types who don't claim to know of the existence of nonexistence of gods, one believer and one not, are agnostics in one respect. What word shall we use to denote their belief or lack thereof? Theism and atheism work just fine. They, too, have Greek roots, from the Greek word for god, theos, and again the prefix that means without, a-.
So we have gnostic theism, agnostic theism, agnostic atheism (weak atheism), and gnostic atheism (strong atheism). I hope by now you have noticed that agnosticism and theism or atheism are NOT mutually exclusive. Further, merely one of these labels may not be sufficient for describing someone. For example, I know that deities with contradictory or logically impossible attributes don't exist. I am a strong atheist with respect to those deities. I do not claim knowledge of the nonexistence of other deities. However, I still lack belief in them because I haven't been convinced that they exist. I am a weak atheist with respect to those deities.
The default position is not believing. This is the default position with respect to everything. You and I don't believe in the world some newb fantasy author somewhere is penning. This is simply because we don't know about it. However, lack of knowledge isn't the end of it. We also lack belief because of lacking knowledge. So, again, the default position is lacking belief and lacking knowledge. The default position does not make any claims about anything. A claim would be, for example, that the newb fantasy author's world does exist, or that God exists, or that God does not exist. Such claims carry a burden of proof. Not claiming anything, simply lacking belief, does not carry a burden of proof.
Everyone of you and I do not believe that there is a teapot orbiting Pluto. Why should we? There's simply no reason. Nobody has ever seen such a thing and it's highly unlikely, though not logically impossible, that one would be out there. But, you know, we can't disprove it's existence. Our telescopes are not powerful enough to resolve such a small object at such a great distance. And even if they could, we'd be searching forever trying to cover every cubic inch of possible space this moving target could occupy. There's no evidence against the teapot orbiting Pluto hypothesis, at least until we get better telescopes, and there no evidence for it either. We are teapot orbiting Pluto agnostics. But again, we not only don't know if there's a teapot orbiting Pluto, we don't believe there's a teapot orbiting Pluto, either. We occupy the default position with respect to this issue, and we carry no burden to prove either the existence or nonexistence of the teapot. We simply don't believe in it.
But what if there was a millennia long tradition of believing in a teapot orbiting Pluto? What if there were teapot orbiting Pluto prophets, priests, and clerics? What if almost all royalty of the past and present believed in a teapot orbiting Pluto? Would you believe in a teapot orbiting Pluto? I'd hope not. Tradition, popular opinion, and authority are not reasons. In fact, arguments that appeal to one or another have informal logical fallacies named after them. Let's say there were also otherwise very intelligent men that devised so called "proofs" that there is a teapot orbiting Pluto. Would you believe them? Maybe, because there we could have actual reasons for believing in a teapot orbiting Pluto. It depends on if these "proofs" succeed in their claim. But what if all of them, without exception, have been shown to fail in their proof? Would failed proofs be legitimate reasons for believing in a teapot orbiting Pluto? No. Are there any other reasons to believe in a teapot orbiting Pluto? No. So we are back where we started. Attempts to sway us one way, to the claim that a teapot orbiting Pluto does exist, have failed. We fall back to the middle, lacking knowledge, and also lacking belief.
Now reread the above paragraph and replace "teapot orbiting Pluto" with "god."
