Game concept
Dungeon
The game also links it as Dungeons.
From the game's encyclopedia
Imprisoned Characters kept in the Dungeon or Stockade suffer from increasing Stress as well as a Health penalty. In time, they are likely to die from Illness or suffer mental breaks.
For Landed Rulers, Commoners are automatically imprisoned in the Dungeon, but can be moved to House Arrest.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Dungeon is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Imprisoned Characters kept in the Dungeon or Stockade suffer from increasing Stress as well as a Health penalty." It is part of Prison. The game also links it as Dungeons. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.