Game concept
Excommunication
The game also links it as Excommunicate, Excommunicated.
From the game's encyclopedia
Characters who are Excommunicated by their Head of Faith are seen as Criminals by other characters of their Faith.
Independent Rulers who have been Excommunicated may have an Excommunication War declared on them. If they lose, they are forced to abdicate to their Heir.
Excommunication can be lifted through repentance, petitioning the Head of Faith, winning an Excommunication War, converting to another Faith, or participating a Great Holy War. For restrictions, timing, and step-by-step recovery, see Recover from Excommunication.
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.
Excommunication is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Characters who are Excommunicated by their Head of Faith are seen as Criminals by other characters of their Faith." It covers Recover from Excommunication. The game also links it as Excommunicate and Excommunicated. 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.