Game concept
Secret
The game also links it as Secrets.
From the game's encyclopedia
Secrets are things that characters do not wish others to know because they are embarrassing, immoral or outright Criminal.
Secrets can be discovered through the 'Find Secrets' Councillor Task for the Spymaster. Once you know someone's Secret, you can Blackmail the character to gain a Hook.
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.
Secret is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Secrets are things that characters do not wish others to know because they are embarrassing, immoral or outright Criminal." The game also links it as Secrets. 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.