//Anything that claims to know something for which there is no evidence, is a belief. Atheists have some mileage in claiming that they do not wish to accept a conjecture for which there is no proof that they find acceptable; and that, that makes all the difference. But the meaning of "Atheism" is a belief/certainty that there is no God, not a belief in there is possibly or probably no God. Although many claim to be atheists yet argue the definition in order not to have to admit theirs is such a belief. There is a difference to that which deists accept, but not such a large one in my opinion. Just a difference of degree and direction.//
While your definition of atheism may be true for many, it is not inclusive of those who came to the conclusion that no gods exist honestly and with much effort of thought, nor with those who have come to no conclusions whatsoever about the issue of a gods existence.
Actually, 'atheist' was originally used by theists as a pejorative term towards those who did/do not subscribe to their particular beliefs in and of who god was/is further complimented with all the assorted sordid baggage they could burden the non-believer with. That is for one the reason I generally don't adopt that as a definitive label to describe my non-theistic view of reality.
http://en.wikipedia.org/wiki/Atheism#Etymology