Both, because atheism and agnosticism are not mutually exclusive.
Except that kinda depends on how you define them, isn't it? As stated above, some people define atheism in a very broad context as any "lack of belief" in a god. Which would make agnosticism a subset (or at least overlap) of atheism. Unfortunately, that's not a very useful definition of atheism since it doesn't help us distinguish anything. IMO, it's just more useful to define them in ways that does make them exclusive sets so as to avoid confusion about where someone actually stands.
Atheism is the lack/denial of belief in a god or gods. Agnosticism is a lack/denial of certainty or ultimate knowledge. An agnostic atheist would be someone who does not believe in a god or gods but lacks absolute certain proof for this (lack of) belief - typically because logically you cannot prove nonexistence.
Most self-described atheists and agnostics are almost always all agnostic atheists.
The problem is, as DSD states, there are a lot of atheists who are absolutely certain that god does not exist, cannot exist, and anyone who believes in god is a moron, and even just accepting the possibility that some form of divine might exist will result in argument from them. These are also the people who are most involved in secular movements and agendas (both politically and socially). Not surprising since they actually care a lot about what is quite obviously a belief.
We kinda need a term for them, don't we. And honestly, the word "atheist" makes the most sense. If someone isn't sure, they're an agnostic. Period. If they are sure that god exists, they are a theist. Period. If they are sure that god does not exist (any god), they are an atheist. All the other combinations were IMO inventions after the fact by people who for some reason want to use different labels in order to avoid criticism of their position or to "fit in" to some larger group I guess.
Dunno. IMO they just confuse the issue because you get people calling themselves agnostics who are clearly atheists (by my definition), and people calling themselves atheists who are clearly agnostic.
a person who denies or disbelieves the existence of a supreme being or beings.
"Not believing in god" =/= a belief. It's a lack of one. The opposite of a belief; a non-belief (which would make that person a non-believer).
Sorry. IMO you are wrong. The definition doesn't say a "lack of belief". It specifically says someone "denies or disbelieves". Disbelief is a belief in the opposite, not a lack of belief in the subject at hand. The prefix "dis" means to reverse the meaning. So a believer is sure god exists, and a disbeliever is sure god doesn't exist. When you disbelieve or deny something you are specifically claiming that it is "not true". It does not allow for the possibility of uncertainty, just as belief does not allow for the possibility of uncertainty. An agnostic both lacks belief *and* disbelief in something.
I'll point out again that the reason I use that set of definitions is because they are useful. The ones you're trying to use are not, and serve only to muddle the issue. Edited, Apr 22nd 2011 2:51pm by gbaji