The answer should be pretty straightforward. A dictionary definition should suffice. And yet many, even among the movement known as New Atheists (Hitchens, Harris, Dawkins, Dennett, Hirsan Ali...) disagree.
Several dictionaries have defined atheism as 'The belief that there is no God' or something along those lines. I know students of Religious Education (and teachers) who still use this as a working definition. Many atheists like myself would - for want of a better expression - be deeply offended by this definition of their beliefs.
But why? Well, here are at least four main objections:
1. It assumes atheism is a belief in a negative rather than a rejection of a belief system. Although there is no consensus, most lucid writers on the subject agree that atheism means 'a disbelief in theism' rather than a commitment to any belief system.
2. It assumes a single clear definition of God. The discussion is clearly framed by monotheism when atheism is defined as a rejection of one God. The theist believes God made man in his own image. The atheist believes man made many Gods in his own image. By the above definition, am I only rejecting one God as an atheist? Which one?
3. It puts a ludicrous burden of proof on what is after all a radical skeptical attitude which requires no proof. No atheist could possibly prove there is no God, but why assume they need to? If a religious worldview asks us to accept that an unseen, unknown deity rules everything, then surely this fantastic claim needs to defend itself. Atheists simply reject it until it is clearly proven.
4. It confuses deism and theism, two radically different approahes to cosmology. The deist believes in God in the loosest sense ("there are unknown forces governing the universe and I don't mind calling them God"). Few atheists take issue with deism, even though it seems to leap to awkward conclusions without proper evidence. At worst, the deist hasn't really thought things through. The real opponent of the atheist is one who claims not only to know that there is a God but to have direct access to the thoughts and wishes of that God through prayer, holy books and rituals. The theist believes not just that there is a God, but claims to know which God holds all the power and how to gain access to him. This inevitably leads the theistic believer to claim knowledge and power inaccessible to the rest of humanity. It is this religious attitude the atheist sets up against: the theist who claims to know the mind of God.
In other words, atheism is what the word suggests: a-theism. A rejection of or disbelief in theism. I don't claim to know whether God exists or not, but I can quite confidently reject the absurd ideologies of those who claim to know the mind of an unseen celestial dictator.
The starting definition could only have been framed by someone who believed in God, and assumed that someone who does not must actively believe the contrary. But luckily the Oxford English Dictionary has updated its definition to something clearer and more palatable to atheists.
Disbelief or lack of belief in the existence of God or gods.
Now I'm with you. The atheist as an unbeliever, a skeptic, a doubter. A doubting Thomas turned blogger, if you will.
To seal the deal, OED need only update their volumes a bit further. Add the words 'afairyism' (the disbelief in fairies), 'asantaclausism', 'abogeymanism', 'aThorism', 'aVishnuism' and a few hundred thousand other a- words to define our daily skepticism of all the Gods, sprites, mythical figures and deities since written record began, which grown up adults lead their lives and pursue knowledge without. When all these 'a-'s are clearly defined, common everyday speech will be ready for a rational discussion of belief and how it affects our daily lives.
That is, until such time as we have all grown out of the need for such words altogether and the beliefs which seem to require such language. We could easily do away with all these 'a-'s by getting rid of silly assumptions that someone who doesn't believe in a myth requires a specific category to define their skepticism. An enlightened future will ditch the need to assume adults rely on mythology for the pursuit of daily activities.
Walking about with a much-abridged Oxford English Dictionary in our pockets will count among the many, many improvements to our world when that time comes. Until then, call me an atheist if you need to, and my first question will be: Which Gods you are an atheist for?