Gnostic Atheist
A gnostic atheist is someone who knows that things like god and whatnot are real. It has nothing to do with actual gnosticism, as that would be an oxymoron. The term "gnostic" in this sense just means that they know. This would differ from the term "agnostic", as an agnostic atheist would just believe there is no god, but would make no claim that we could know if god existed or not. A regular atheist would be someone who doesn't claim to know if god doesn't or does exist, they would just not believe.
I can call myself a gnostic atheist, as I can say for certain that things like ghosts and the like do not exist.