Well, words mean however people use them, but going by word roots, "atheist" comes from "a" (not) "theist" (beliver in God). Agnostic comes from "a" (not) "gnostic" (knowing). It doesn't mean "I don't know," rather it means one believes the answer is unknowable.

If you agree with this statement: "I currently do not believe I have reason to believe God exists," you are an atheist.