Atheism
The lack of a belief in the existence of a god or gods. This usually also means a lack in a belief in an afterlife, the soul, the supernatural, or any sort of ultimate reality outside of the physical one.

For the atheist, reality is limited to reductionist empiricism, morals are usually relative or determined by the rules of conduct that make civilization possible, life is it's own purpose, and the truth or falsity of religious beliefs is apparently subject to evidence (though there are no specifications as to what these are).
Athesim is not, despite many claims here, the default belief of all life or humanity at birth. To say that babies and animals are atheists is silly simply because we cannot ask them, and some indeed even argue that they are aware of the presence of "god" within them. Also, the fact that nearly every culture around the globe has, in some form or another, developed some mode of religious belief goes to show that religion is a much deeper part of the human experience than this particualr brand of atheism will admit.