I can hear the groans already. "Oh yeah, just what the web needs. Another one of these posts."
Yes, it's not the only post of its kind by any means, however the question is still often asked. I was asked a version of this question just the other evening and no doubt wih the looming (and horribly misnamed) World Youth Day on the horizon here in Sydney, I'm almost guaranteed to be asked again.
So straight into it. What is an atheist? The way I'm currently explaining it is something like this.
First, we must define terms. Atheism, simply put, is "not theism". AskOxford defines theism thusly:
theism
/theeiz’m/
• noun belief in the existence of a god or gods, specifically of a creator who intervenes in the universe. Compare with DEISM.
— DERIVATIVES theist noun theistic /theeistik/ adjective.
— ORIGIN from Greek theos ‘god’.
So essentially, If you can't say with any degree of certainty that a god or gods exist, you are an atheist. Really. Those of you who self identify as 'agnostics', well, you're atheists too, because you do not express a belief in god or gods.
This is really not a difficult concept to grasp, however it's often misunderstood. The common canard that athesists don;t beliee in anything therefore cannot have any moral sense is laughable in the face of a correct definition. Atheism is a negative response to a single question:
"Does god exist?"
That question implies no moralistic interpretation. There is no baggage. It's a yes/no/maybe answer and nothing else. Morality does not follow.
Of course, pitiful humans can always be relied on to fuck up good logic, and this defintion is no different. Due to the slippery nature of theistic claims, it's not always a black/white, yes/no, on/off situation*
Definitional question are relevant. For instance I have a good friend who expresses an actually moderately rational belief in a "prime mover", some kind of undefined intelligence which somehow brought our universe into being, set the laws of physics and so on. This being is, by definition, outside the universe and does not intrude into it, and is, for all intents and purposes, non-detectable, non-intervening and, in the strictest sense of the term, non-existent in our universe. Is this person a theist?
I'd say no
Is this person an Atheist?
Well, yes.
I'd say my friend has simply found a position which rationalises his own irrational gut-feeling that something brought the universe into being. The position in non-assailable by normal testing, though it does fall to Occam's Razor, which states the simpler explanation is the more likely. And an intelligent being that exists outside the universe is more improbable than current cosmological theories into the ultimate origin of our universe**. But he's still not expressing belief in an interventionist, extant god.
There's also the "degrees of certainty" issue
OK, so what about someone who says, when asked, "God probably exists, but I'm not sure".
Theist
To say "probably" is just bet-hedging. This kind of person believes. What about "God probably doesn't exist, but I'm not sure"
Atheist. A weak Atheist, but still. Cannot be classed as a theist, so logically must be the opposite.
Some people may disagree, well, I've got two words for you, Buddy. See if you can guess what they are.
(clue: it could be "comment please")
* Well, for me it is. But I'm a special case
** If you have to ask why, I'll ask a question in return: where did that intelligence originate? It's turtles all the way down.