Is athiesm a belief? What is "God?"

[Reasonable]

Well, one good, key point there, what definition of God are we using and what does it mean?

For example, it may interest you to know that if you read Richard Dawkin's The God Delusion, he makes clear he isn't against the idea of a creator per se (and indeed this cannot be disproven fully at this point, although recent material from Stephen Hawking might change his view there) but the idea of God as defined in our own religious texts - the Bible, etc.  He is also of the view that the key difference is whether by God you invoke supernatural and the afterlife or whether you mean a highly, highly evolved and capable entity.

For me, God relates specifically to our religions plus the idea of the supernatural.  My atheism is not a belief so much as an acknowledgement that the idea we were created by a God directly, that the Earth was created whole 10,000 years ago, that when we die we magically wake up in a nice garden, etc. are not only not supported by any evidence but are, in many different ways, disproved by the available evidence.

Now, does that mean I wouldn't consider the idea of creators, etc.?  No, I'm perfectly happy to consider that.

The analogy I'd give is the old Gary Larson cartoon with an old guy with a beard baking the World in his oven.  I don't believe in that old guy.  Replace him with an alien entity divorced from any supernatural trappings, and sure, we have no evidence one way or the other.

Certainly, SF writers have come up with many ingenous ideas for creators and supremely powerful beings that don't involve God or the supernatural, which are feasible and which currently couldn't be disproved.

For example, suppose I was to posit the idea that an alien race sent out tiny probes millions of years ago designed, as an experiment, to introduce simple chemical life to suitable worlds they came across, and that one happened to pass the Earth, judge it correct, land, ensure basic life began to evolve, then take off and head away, leaving the rest to history and the processes of chance and evolution?

The above is certainly feasible, doesn't involve a supernatural God and I doubt could be easily disproved at this point.

What would we think of aliens with such capabilities?  Gods?

It's important to consider the root and initial development of words.  Atheism and Agnostic, so far as I'm aware, are words that specifically came into being regarding our own definition of God and our own religions.  Clearly today they can be taken differently, and our views have changed much, but in principle I'd say Atheism means not believing in any Earth generated religion involving the supernatural and Agnostic is being unsure specifically with regard to those religions.  However, Atheism doesn't mean a disbelief in other potential creators or options.