An oubliette is a dark pit, a dungeon with no doors. Its name is based on the French word for "to forget," because people left in the pit were supposed to be forgotten. The only obvious way in or out of an oubliette is through a hole in the ceiling, which is also the only source of light for any of its occupants.
In the film Labyrinth, Sarah finds herself trapped in an oubliette after telling the Helping Hands to drop her down (they asked her which way she wanted to go, she chose down). Hoggle helps her out, as he has access to a door that can be attached to the oubliette. This door can either lead an individual out of the oubliette, or into a broom closet.