@Miro: what if the earth orbits The Flying Spaghetti Monster and the Monster's glory is so bright we mistake it for a star and call it the Sun?
@original topic:
My definition of "god" is whatever one ascribes as the final source of comfort and trust. For instance, I might put my faith in money and make it my god. No matter what happens, if I have money, everything will turn out all right. No medical condition can't be solved by paying the doctors! All I need, my clothing and shoes, food and drink, house and home, etc. is given and sustained by my money.
So, everyone has a god by that definition. Many nowadays would put all their respect, love, and trust into Reason: at the end of the day, we can figure truth out on our own and solve all the world's problems--all we have to do is think for ourselves and not let the religions/ideologies do it for us.
Now, the entity to whom I (at least in theory, often times it's easy to get distracted) ascribe all my respect, love, and trust is Jesus Christ. He is my "god".
You could define god as a high-in-the-sky ephereal spirit thingy, but if you did, you would be describing something that I don't believe in either. My god is a human.
(I could get a lot more technical, but this is a forum post, so that's the essance of it)