When an element of a game's state is hidden from a player, the component or element of the component that's currently tracking it is not currently visible to that player. Openness and closedness are alternate terms for visibility and hiddenness.
Hiddenness is not identical to mystery or opacity. Things that are mysterious are by definition hidden, but things that are hidden are not necessarily mysterious. When something is hidden but was visibile at an earlier time, that counts as an instance of opacity rather than mystery.
For more information about the unique problems caused by non-mysterious hidden information, see Hidden Trackable Information.