Game concept
Duchy
The game also links it as Duchies.
From the game's encyclopedia
A Duchy is a Title one Tier below a Kingdom and one above a County.
The Holder of a Duchy has De Jure rights to its constituent Counties and can have Counts as Vassals.
Duchies may be dormant and have no current Holder. Such Title Succession can be created by a Ruler who controls at least half of its Counties.
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.
Duchy is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Duchy is a Title one Tier below a Kingdom and one above a County." It is part of Title. The game also links it as Duchies. 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.