A dungeon with an opening only at the top; a secret place of imprisonment.
There are many variations on this "pit prison" concept. In general, a prisoner was lowered into the windowless dungeon, and then the trap door at the top was closed. Food might be lowered to the captive through the door...or not. In some variations, spikes lined the floor and the prisoner was thrown in. Castles with oubliettes include Leap Castle in Ireland, Pembroke Castle in Wales, and Newark Castle in Nottinghamshire.
Thankfully, some tales of oubliettes may be exaggerations: some chambers that match the general description may have been storerooms, or latrines. The oubliette of the Bastille might have been used for ice storage.
"The X-Files" episode titled "Oubliette" was about a child abduction.
Etymology: From French oubliette, "forgotten place", from Middle French oublier, "to forget, to show negligence". The word was in use in French since the late 1300s, but the first English use is stated to be in Walter Scott's "Ivanhoe" in 1819: "...the place was utterly dark - the oubliette, I suppose, of their accursed convent, and from the close, stifled, damp smell, I conceive it is also used for a place of sepulture."