Oubliette

(n.) A dungeon with an opening only at the top, found in some old castles and other strongholds, into which persons condemned to perpetual imprisonment, or to perish secretly, were thrust, or lured to fall.

Thesaurus Entries

POW camp, black hole, borstal, borstal institution, bridewell, brig, bucket, caboose, calaboose, can, cell, chokey, concentration camp, condemned cell, confine, constrain, death cell, death house, death row, detention camp, federal prison, forced-labor camp, gaol, guardhouse, hoosegow, house of correction, house of detention, immure, incarcerate, industrial school, intern, internment camp, jail, jailhouse, jug, keep, labor camp, lockup, maximum-security prison, minimum-security prison, oubliette, pen, penal colony, penal institution, penal settlement, penitentiary, prison, prison camp, prisonhouse, quod, reform school, reformatory, sponging house, state prison, stockade, the hole, tollbooth, training school