It came up on youtube.com, a purveyor of .... well ... whatever the hell video you upload, that a video called "Atheist" became immensely popular and then suddenly disappeared from the most-viewed list. Conspiracy was presumed but unproven. The video made a couple of simple, but apparently very upsetting statements, regarding atheism. The first was that an atheist is a person who doesn't believe in any gods. There were quotes from the bible, where it is stated that atheists are, by their nature, "fools" and "evil". This was followed by a list with images of nice, smart atheists.
The reply videos were entertaining, if nothing else can be said of them. The best contained the most common of flawed premises: the redefinition of atheism. For some reason, among those who are not atheists, it is more convenient to label an atheist as "one who denies the existence of God".
While these definitions seem similar, there's a glaring gap in the logic between them.
If you flip a coin and hide the result from me, you might ask "Do you believe I flipped heads?". I can answer to this question, "No". You might then state, "Ah, then you believe I flipped tails." I would then say, "No, I don't believe that either."
That is the key difference between the two. The definition that atheists use for atheism is, "I do not believe your proposition that god exists". The believer then says, "Ha, you therefore believe that there is no god. Now prove it!" This always leads to a hearty laugh from the atheist who invites the believer to prove unicorns don't exist. The believer gets offended, because the atheist is being a bit of a prick, and goes off in a huff. The point, however, is that the atheist never claimed to know that god doesn't exist, only that he didn't see any reason to think so.
I'm not sure how this definition problem happens, but I can make some educated guesses.
The first guess is that "the dictionary says so". Various dictionaries have various definitions and the writers of said dictionaries have their own biases. Some have one definition. Some have the other. Mind you, a definition of atheism that says, "denial of god's existence" is rather like an unheard of definition of christianity as, "belief in mythology". It assumes the person being defined to be wrong. Presumably we only do this to atheists on account of atheism being quite in the minority.
The second guess is a matter of logical consistency. When confronted with a demand for evidence of what is gathered from faith, the believer prefers to put the non-believer on the same ground. If an atheist can be defined as a person who says, "there is no god", it can then be demanded of the atheist that he produce evidence. Then we get the prickish bit about the unicorns.
(And no, that's not an agnostic. "Agnostic" means something else entirely and every dictionary I've ever checked agrees at least on that point.)
So, there you go. Atheism just means "not believing", not "believing in the opposite". Come on over. We have lower divorce and incarceration rates, no holy days to get you out of bed on the weekend, and you get a 10% discount on drinks at all casinos and strip joints (when you show your card).