Correct me if I'm wrong, but the belief that there is no god is a belief...if you want to get that specific. I think no belief would be...agnosticism...not necessarily believing anything in particular.
Did you read what I wrote?
Atheism and theism refer to forms of belief. Theism is the belief in a religion, atheism is a lack of that belief. not belief in the opposite claim.
not believing in god =! believing there is no god
a/gnostic refers to forms of knowledge. If you say "i dont know" to the question, "is there a god?", you still lack the belief in god.
knowledge is a subset of belief. you can believe in something and not claim to know it 100%
agnostic means i don't know / it can't be known depending on your definition.
Most theists are gnostic theists, i.e. they believe in god and claim they know it for a fact.
my cat is atheist
You know what your cat thinks?
Its not possible for my cat to have a belief in god. No belief in god = atheist.
Fatcat, whomever makes a positive statement ( I believe xyz.....) has a burden of proof on them.
Religious people make a positive statement, and justify it.
Athiests also make a positive statement. They also justify it, meaning they believe something.
Its really the agnostics that can claim they dont believe in anything dogmatically
Atheism is a responsive position to claims of god. Atheism is not a positive statement.
Now if you want to use a different definition, then fine, but under standard etymology the A suffix usually means lack of, amoral for example is not the same as immoral
Now I don't know any atheist who claim to know for an absolute certainty that no god exists, much like I can't claim to know to an absolute certainty there isn't a unicorn in existence on some Micronesian island somewhere, but in day to day uses of knowledge, much like I claim that I know there is no such thing as unicorns and leprechauns, I claim to know there are no gods, as I have not seen sufficient evidence to believe such a claim, but like you said, it would require some sort of proof or evidence to claim to 100% certainty that there are no gods, namely an absolute knowledge of the universe/existence, which I don't have and likely never will.