I hope you find this to be a good definition!
Copied right off of my facebook group (http://www.facebook.com/group.php?gid=127560480616199 it even has a cute little picture of a cartoon owl) :
The "big owl" definition is, in my opinion, the simplest, most accurate, and best definition of an atheist. Not all atheists think "there is no god" but rather just lack the belief in god. You can be an atheist without declaring there is no god. You just need to lack the belief in god. The "big owl" definition would mean babies and people who have never heard about god are atheists. In a sense, it is the default position until hearing about god. It includes pretty much everyone other than theists.