Game concept · All Under Heaven
Japanese Emperor
The game also links it as Japanese Emperor's, Ten'nō, Ten'nō's.
From the game's encyclopedia
The Ten'nō is the official Emperor of all Japan, though true power is generally in the hands of the Ruling Regent, who administers the state on their behalf.
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.
Japanese Emperor is an entry of Crusader Kings III's in-game encyclopedia (All Under Heaven). The game explains it: "The Ten'nō is the official Emperor of all Japan, though true power is generally in the hands of the Ruling Regent, who administers the state on their behalf." It is part of Ceremonial Monarch. The game also links it as Japanese Emperor's, Ten'nō and Ten'nō'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.