From the game's encyclopedia
Imprisoned Characters can be Tortured, damaging their Health or even maiming them in different ways.
Torture can relieve Stress for Callous and Sadistic Characters, but causes Stress for those of a kind nature.
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.
Torture is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Imprisoned Characters can be Tortured, damaging their Health or even maiming them in different ways." The game also links it as Tortured and Torturing. 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.