Known unknown
/nəʊn ʌnˈnəʊn/ (n.)
As famously articulated by Donald Rumsfeld, something you know you do not know. Depending on your approach to epistemology this is could mean different things. A reductionist would say “this is simply an existing fact about the universe, past, future or present, that already exists, but which I do not yet have in my possession” — because a reductionist would see the universe as a fully solved, just not fully revealed thing — like an uncompleted crossword. Thus, a “known unknown” is a measure of one’s potential disadvantage against a hypothetical other to whom that thing has been revealed. The cosmos contains a certain, finite amount of facts — “potential knowns”, and the “known unknowns” are simply those of them that are not in your possession right now. But, good news — a machine learning has almost solved chess and go, the remainder of the universe isn’t far off, so some artifical intelligence will be along shortly to derive all extant knowns, at which point the universe will wake up, a “game over” sign will flash up on the screen and it will be time to go home for tea.
A pragmatist would say a “known unknown” is some random and unexpected shit that we can see happening without warning.
See also