An atheist is someone who sees absolutely no evidence that the universe was once created, or is currently managed by, a conscious intelligence (aka "God"). Of course, it is impossible to prove that no such thing exists, but if you see absolutely nothing to make you believe in it's existence, then it is not an unreasonable assumption to make.

The universe appears to be completely chaotic and random, but there are a few physical laws that do seem to apply which forbid certain things from happening, and make others inevitable. This configuration does not require a deliberate thinking being to make it happen. End of definition.

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There are variations on this theme, (perhaps an infinite number of them), none can be ruled out, either by itself, or in combination with others. I will list only a few:

There is a God who created the universe, but he no longer takes an interest in its operation. Like any perfect machine, it runs itself.

He may be busy doing something else, or resting, or dead.

God does run the universe, but he is not omnipotent, omniscient, or immortal. (Or any combination of those you like).

God may indeed be real and active, but he is busy doing really big things, not meddling in our lives or engaging in any activities meaningful to us.

There may be a God, but he is not a divine, supernatural being in the usual sense we use the term. He may be just a highly advanced being with enormous, but finite, powers who may not even be aware of our existence, any more than we are aware of the presence of our own intestinal flora.

There is more than one God, perhaps a hierarchy of them. Don't laugh, this is what most religions throughout history have believed, and you have no reason to think they are all wrong and you're right.

I could go on...but it would all be just speculation. The only thing we know for sure is that IF there is a God, we know absolutely nothing about him because he has never bothered to let us in on it.