Someone who has a lack of belief in God/gods. One who has looked at all the evidence and believes the bible to not be a reliable source of evidence for how the universe was created and disagrees with its morals. They understand that the Bible endorses slavery, encourages intolerance, and produces an irrational fear of "hell", a scary fiery place where all those who do not believe will be tortured forever. An atheist believes in his/her own morals and understands that this is their only life and it should not be wasted. They do what they can to make the world a better place to live by referring others to the reliable sources of science and reason. They understand that no one in over 2000 years has proven that there is a god or miracles. They have no loyalty to evolution because Atheists understand that the two are not intertwined. They accept challenges from religious people to read the bible and research the "facts" in it. Finally, they understand that two hands working accomplish much more than 1000 clasped in prayer.
Christian: "I had a headache and I prayed to God and it went away. Explain that!!"
Atheist: "Did you do anything else about it?"
Christian: "Oh yeah, I took that stupid "Aspirin" stuff that scientists have proved to work well to remove headac----whoops."
Atheist: "There you go."