dungeon vs oubliette

dungeon and oubliette both are nouns.

Word	Noun	Adjective	Verb	Adverb
dungeon	Yes	No	No	No
oubliette	Yes	No	No	No

As nouns, oubliette is a hyponym of dungeon; that is, oubliette is a word with a more specific, narrower meaning than dungeon:

dungeon: a dark cell (usually underground) where prisoners can be confined
oubliette: a dungeon with the only entrance or exit being a trap door in the ceiling

dungeon (noun)	oubliette (noun)
the main tower within the walls of a medieval castle or fortress	a dungeon with the only entrance or exit being a trap door in the ceiling
a dark cell (usually underground) where prisoners can be confined