Game concept · All Under Heaven
Shōgun
The game also links it as Shōgun's.
From the game's encyclopedia
A Shōgun, or Dictator, is the de facto Sōryō Government ruler of all Japan, who has consolidated their position to the point their succession is hereditary, and reigns in the Ten'nō's stead.
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.
Shōgun is an entry of Crusader Kings III's in-game encyclopedia (All Under Heaven). The game explains it: "A Shōgun, or Dictator, is the de facto Sōryō Government ruler of all Japan, who has consolidated their position to the point their succession is hereditary, and reigns in the Ten'nō's stead." It is part of Ruling Regent. The game also links it as Shōgun's. Patch 1.20.1.2, from game/common/game_concepts/tgp_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/tgp_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.