I'm answering the thread in general from the title, so I don't know where it's been.
Atheism is a term that assumes theism as the default. We don't usually name the lack of something unless we assume that it is a needed and proper thing that people shouldn't lack. Take a group of people and demand that they divide themselves by their favorite god. Atheists are among those that didn't choose. More specifically, atheists are the people that refused to participate on the grounds that they don't know that any gods exist. They aren't properly a named group. The question they answered isn't the same one as the other groups answered. In very real social terms, atheists are the ones refusing to play a game that religionists demand they play.
I suppose I'm a de facto atheist, but I refuse to identify with the term because its framing suggests deviance. On the contrary, it's actually the norm in some circles. Since when are we required to participate in groupthink about deities? While it is true that some societies have required it of us for centuries, that's not a good thing. I'm not deviant or rebellious. I only seem such to some because they wrongly assume that religion is required. The question it answers presumes an authority which I never ceded. I am not required to answer the question of deities at all, and usually, I prefer not to answer absurd questions. I'll just leave that one blank. It doesn't deserve an answer. It's a false dichotomy, for one. People aren't entirely temporally consistent, and sometimes their answer will depend on context. We assume by the way we attribute it that people aren't sometimes an atheist and sometimes not, but the fact is that this is actually a fairly common condition. To top it off, you don't even have an opinion all the time unless you're constantly thinking about it.
My definition of atheism would be: "The practice of answering a ridiculous question that doesn't even deserve consideration with a negative, ignoring the fact that the question itself is illegitimate and manipulative." The real question intended isn't about whether gods exist or not. It's about which one you grovel to. Their existence or lack thereof is irrelevant to any meaningful discussion of religion. The dividing lines aren't even where we pretend that they are. There are people who don't believe in the factual existence of gods who nonetheless identify with a theistic religion, and there are people who believe in some vague concept of a deity who don't follow a religion. There are even people that believe one way in some situations, and another in others. This particular episode of "separate and classify" doesn't clarify anything meaningful. It's mostly just an identity crutch. In that, it's not notably different from religion. It makes much more sense to me to just be a non-participant and decline to discuss the matter.
That said, I can't resist discussing the matter because I find it fascinating.