That's a trick question.
Are you asking what an atheist may be from their own point of view? Or what others might consider to be one.
Extremist Christians believe that if one does not believe in their "God" one does not believe in god. Some people can make a good case for the idea that if one believes in a Christian "God" one is an atheist, the belief being that that "God" clearly doesn't exist.
Are Buddhist's atheists? Not from their point of view, but certainly from a Christians. So who do you ask, the Buddhist about himself or the Christian about the Buddhist?
"Atheist" is one of the words that our society has garbaged up pretty badly. Others are "love" and "god".
Arf