Game concept · Friends and Foes
Feud
The game also links it as Feuds, Feuding.
From the game's encyclopedia
Sometimes, a rivalry can grow into a vendetta against a particular House.
Killing, Torturing, cuckolding, robbing, and winning Wars against members of the target House will increase your House's score, while falling victim to these actions will reduce it.
While a Feud is ongoing, all House members receive a Hostile Schemes bonus against members of the target House, and will be more likely to target its members.
When the Feud ends, your House will be rewarded based on the relative damage inflicted on its enemies, ranging from Pathetic Family to Formidable Family.
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.
Feud is an entry of Crusader Kings III's in-game encyclopedia (Friends and Foes). The game explains it: "Sometimes, a rivalry can grow into a vendetta against a particular House." The game also links it as Feuds and Feuding. Patch 1.20.1.2, from game/common/game_concepts/01_bp1_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/01_bp1_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.