Well, discluding all of the religious beliefs in the world.. The definition of God is a very hard one. I would, usually, say that a God is a being that can change the world in some way at whim.
Yet, that isn't a very good one. A single person can change the world at whim. Similar to light (and other such things) a person takes every possible route at every possible moment they can. However, each one is separated into different dimensions, which we don't know how to get to. This is also why time travel backwards is impossible, since the past is also fluid, which also contributes to the infinitely possible things that can happen at every moment. So, that said, every living organism is a God, according to that definition.
So, I think the best definition of a God is a being that can bend the world at whim, and is worshiped. The whole worshiping thing makes you special.