Jesse

I would say that they are not directly interchangeable but they do overlap eachother in various ways.

An atheist is literally someone who is godless. That's not to say atheists assert, "There is no god," though some of them do, but to say that the general condition of not having a god. There is another way of looking at it. If you accept "god exists" as a true proposition, you are a theist, and if you do not accept it as true, for any reason whatsoever, you are an atheist.

One type of atheist is the "positive atheist." The word "positive" is a reference to having accepted a belief about the truth or falsity of the proposition "god exists." As such, a "positive" atheist is one who thinks "god exists" is a false proposition. Positive atheists make up only a small portion of all atheists. The other subset of atheism is "negative" atheism. The word "negative" only means "not positive."

You could be an agnostic atheist. That is a person who thinks knowledge of the existence or nonexistence of god is unattainable (hence agnosticism) and because it is unattainable they have no evidence that would justify them accepting the proposition "god exists" as true (hence negative atheism).

You could be a noncognitivist atheist. That is a person who thinks the word god has not been adequately defined. This lack of definition makes the term god essentially meaningless to them, which prevents them from forming an intelligible idea of such a concept (i.e., they idea of god has no cognitive substance, hence noncognitivism.) To them, to accept the proposition "God exists" is just as meaningless as you accepting the proposition "Unie exists." (Unie is left undefined. Try to believe a Unie exists and you'll see how they feel about "god.") Thus, they don't accept that proposition as true (hence negative atheism).

One can be a negative atheist who is not agnostic and not noncognitivist. This kind of atheist thinks the word god has been defined well enough to form at least a vague idea of what the word god would refer to. As for agnosticism, it implies that you believe (i.e., have accepted as true the proposition) "knowledge of the existence or nonexistence of god is unattainable." This atheist might believe knowledge is attainable. This atheist might not believe knowledge is attainable but also not believe it's unattainable. Essentially, this kind of person is godless merely because they've been presented no good reason to accept the proposition "god exists" as true.

One can be a skeptic. A skeptic of the proposition "god exists" is someone who doubts the proposition is true. Skepticism applies to all atheists, regardless of whether they are positive or negative. Skepticism also applies to many theists or deists. They might have faith that "god exists" is a true proposition but they are skeptical about whether it really, truly, is true. They are not too different from a negative atheist except they take that one step beyond where there reason took them by having faith but they're reason makes them skeptical about whether they should have taken that extra step. Most atheists who were once theists or deists have probably experienced this.

Rationalism is a system of thought that emphasizes the role of reason in obtaining knowledge, in contrast to empiricism, which emphasizes the role of experience, especially sense perception. Most people combine rationalism and empiricism in their worldview. The combination is the emphasis of the role of reason in obtaining knowledge through logic and empirical evidences. It should be fairly easy to see how the terms rationalism and empiricism can overlap skepticism, agnosticism, noncognitivism, and the various kinds of atheism.

There isn't much grey area in the definitions, presuming they're defined correctly. Many people have become hopelessly confused as to what the terms mean and how they relate to eachother though. This confusion was, in my opinion, mainly caused by apologists and theologians. Apologists and theologians have contorted the definitions in various ways to further their ideological goals.

They misdefine atheism. They make it out as though atheism consisted of only people who believe "god exists" is a false proposition. Most atheists do not hold that position so it misrepresents them. When theologians and apologists portray atheism this way it leads the audience toward the implication: the atheist thinks he or she knows how the universe was created because it's only with this knowledge that one could say god didn't do it. When the atheist is unable to explain the origin of the cosmos the audience is led to think that the atheist is retreating from his or her position. This strengthens the faith of those listening to the apologist or theologian and that helps ensure more conversions and book sales, and advertisement revenue if they debate online. (Many theists simply don't know better and unknowingly spread this false information.) In actual fact, the atheist was misrepresented and didn't change his position at all--the retreat is an illusion.

You might have heard that atheism and nontheism are the different. This is also an illusion. The prefixes "a-" and "non-" do not have different meanings. Whenever someone says to me that atheism and nontheism are different, I challenge them to explain the difference between atypical and nontypical, and once they tried to offer an explanation I ask them whether that explanation was atypical or nontypical. I've found this to be a very effective way of shutting these people up.

Because the apologists and theologians have been misdefining atheism, this causes problems with the word agnosticism. If all atheists think they have knowledge--as the illusion goes--then all agnostics must be those without knowledge. This is partially true but partially untrue. Agnosticism is about the ability to attain knowledge, not about whether it has been obtained. The difference is glaring. Just because it is possible to get something doesn't mean you have it. The negative atheist who is not an agnostic (who thinks it is possible to get knowledge but doesn't have it) is now called an agnostic and not an atheist. This is a complete reversal. The reversal is caused by the misdefinition of atheism.

I hope this helps you understand how the terms relate to eachother and how they have been confused with one another. If I didn't explain clearly enough then let me know which part(s) you didn't understand and I'll try a different way of explaining.

As for the subject of secular humanism, I just noticed that American Humanist Association states, "Humanists recognize nature as self-existing." This is a positive atheist statement, in which case you would not belong. This contradicts the statements of the Council for Secular Humanism. The person who started the Council for Secular Humanism used to work at the American Humanist Association. I think it's safe to say that the guy didn't agree with the American Humanist Association in limiting secular humanism to only positive atheists. I would support the more inclusive definition of secular humanism, but I'm not the arbiter in the conflict. To be honest, I think it would be better to avoid calling yourself a secular humanist.