If something is in principle, by definition, *unknowable*, then it seems to be in principle, by definition, *nonexistent*; is there a term for this thing?
Partially, it is Occam's Razor principle--which is to use the simplest theory that explains all the facts. If there is no way to perceive (know) the thing, then it is simpler not to include it in our model of reality.
Trying to google it made me realize that even philosophers didn't have much use for a term for things that can't exist in any space.
I propose borrowing the buddhist term Śūnyatā