Game concept
Vizier
The game also links it as Viziers, Vizierate.
From the game's encyclopedia
A Vizier is a type of Diarch that Clan Rulers may voluntarily appoint to help boost their Income. They have less to Borrowed Powers but can be very difficult to remove.
Viziers grant their Liege bonus Tax Jurisdictions scaling to their Stewardship, and increase the Aptitude of Tax Collectors according to how much the Scales of Power favor them.
Crucially, Lieges can drain Viziers of their GoldGold via the Mulct Vizier Interaction.
Being empowered civil servants, Viziers are not considered a prestigious position for Nobles despite their influence. Their successor is picked according to Vizier Succession Score.
As a Ruler can only have one Diarch at a time, they'll remain in the event that a Regency would occur.
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.
Vizier is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Vizier is a type of Diarch that Clan Rulers may voluntarily appoint to help boost their Income." It is part of Diarch. It covers Vizier Succession Score and Vizier Extravagance. The game also links it as Viziers and Vizierate. 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.