I suggest you use the definition American Atheists use.
"Atheism is not a disbelief in gods or a denial of gods; it is a lack of belief in gods. Older dictionaries define atheism as "a belief that there is no God." Some dictionaries even go so far as to define Atheism as "wickedness," "sinfulness," and other derogatory adjectives. Clearly, theistic influence taints dictionaries. People cannot trust these dictionaries to define atheism. The fact that dictionaries define Atheism as "there is no God" betrays the (mono)theistic influence. Without the (mono)theistic influence, the definition would at least read "there are no gods."
Why should atheists allow theists to define who atheists are? Do other minorities allow the majority to define their character, views, and opinions? No, they do not. So why does everyone expect atheists to lie down and accept the definition placed upon them by the world’s theists? Atheists will define themselves."
"The only common thread that ties all atheists together is a lack of belief in gods and supernatural beings."
http://atheists.org/activism/resources/what-is-atheism?