Game concept
Domain
The game also links it as Domains.
From the game's encyclopedia
A Ruler's Domain is the collection of all Holdings that they personally own.
The fairly static Domain Limit means that Rulers have to rely on Vassals to increase the size of their Realm beyond a certain point.
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.
Domain is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Ruler's Domain is the collection of all Holdings that they personally own." The game also links it as Domains. 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.