I was so proud of myself when I saw the title of this episode and said, "I know what that word means!" I wanted to write a review to tell you guys because I find it so ironic that the episode is called this, but the word is never even mentioned nor explained. But it completely fits the episode!
"Oubliette" is French for 'dungeon' and can also be connected to another French word, "Oublier," which means 'forget.' In other words...forgotten dungeon. An Oubliette was a Medieval torture chamber in which prisoners were thrown into a room below the floor. This room had no windows and the only way out was by a door high in the ceiling, the very door in which they were cast into. The prisoners in there were then intentionally forgotten about and many of them died from starvation or from going mad.
I love that the writers did their research and I absolutely LOVE this episode! A wonderful Mulder-centered episode and if that wasn't enough, it is also beautifully written.
(PS: I'm sorry if I got some of the French definitions wrong. I don't speak French, but I know all of that through research)