Game concept
Imperial Family
The game also links it as Imperial Families, Imperial.
From the game's encyclopedia
The Imperial Family is the top House in a Bureaucratic Realm.
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.
Imperial Family is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The Imperial Family is the top House in a Bureaucratic Realm." It is part of Noble Family. The game also links it as Imperial Families and Imperial. Patch 1.20.1.2, from game/common/game_concepts/07_ep3_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/07_ep3_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.