Websters defines atheism as...

2 a : a disbelief in the existence of deity b : the doctrine that there is no deity

That's why there are two different words for belief in the non-existence of god (atheism), and lack of belief in a god (agnosticism).

The roots of the word agnosticism are a (without) + gnosis (knowledge).

The roots of the word atheism are a (without) + theos (god).


Agnosticism is the acceptance of a lack of knowledge of god, and atheism affirms the non-existence of god.

Someone who acknowledges that they don't know one way or another if there is a god may call themself an atheist, but they are really an agnostic.