I recently heard a good definition of atheism:
The person defining it this way expanded it to include another idea:
Atheists do not insist that they know God does not exist. More precisely, atheists have no reason to believe in God.
Evidence has not been presented in an amount sufficient to convince atheists of the existence of a higher power. It is not necessarily true that atheists are closed-minded to any or all offers of such evidence. After evaluating the evidence offered thus far, the atheist simply remains unconvinced. Not rebelliously unable to be convinced, and not stubbornly unwilling to be convinced. No. Rather, the atheist is sincerely choosing to remain faithful to what he or she knows to be true, often against great social pressure to give in and profess belief in a supreme being.