These definitions seem to be attributed to Donald Rumsfeld, former US Secretary of Defense. The attribution is usually derogatory, e.g. “Rumspeak”. But I think its great …
Known knowns – things we know we know
Known unknowns – things we know we don’t know
Unknown unknowns – things we don’t know we don’t know
Rumsfeld would add another relevant, telling category later: the unknown known, the kind of thing you think you know, based on information that seems solid but which in reality isn’t very solid at all. It’s the thing you think you know but that you actually do not.
Here is a video of Rumsfeld describing his uncertainty principle.