Atheist (n.): a person who does not believe in the existence of a god or supernatural being.
"Atheism, in its basic form, is not a belief: it is the absence of belief. An atheist is not primarily a person who believes that a god does not exist; rather, he does not believe in the existence of god." (Smith, 1989) This is an important philosophical point because an atheist (or anyone else) cannot deny the existence of something that simply does not exist. For example, a person cannot claim that another individual is denying the existence of Spiderman if Spiderman does not exist. Spiderman is simply a word referring to a comic book action hero concept. Spiderman only exists through secular (i.e., stuff of this world as opposed to the spiritual world) things such as movies, actors, posters, comic books, imaginations and so forth and does not exist in reality as a real person. Atheists believe that God is also just a word referring to something that does not exist in reality. This word "God" only takes on meaning through secular things such as churches, testimonies, imaginations, behaviors of followers, Bibles and so forth. Without these secular things, the God concept would no longer exist just like Spiderman would disappear if all the Spiderman movies, comic books, memories of Spiderman and so forth were suddenly erased. In sum, atheists believe that both Spiderman and God do not have any separate existence independent of these secular things.
Atheism: The Case Against God (1989) by George H. Smith, p. 7