Atheism refers to the rejection of belief in any god(s) or spiritual realities.  The physical world which is observable by the five senses constitutes the totality of what is real.  There are several forms of atheism, differing primarily on an optimistic outlook on life (the possibility of hope, purpose, meaning, and knowledge) versus a pessimistic outlook (denial of these things).

These forms of atheism are explained in the following links:

Naturalism	| an optimistic version of atheism
Marxism	| clearly a form of modernism
Nihilism	| "That which does not kill me only serves to make me suffer." -Eric Marcus
Existentialism	| a coping mechanism in the face of nihilism

Atheism Media Reviews