Game concept
Culture Head
The game also links it as Cultural Head.
From the game's encyclopedia
A 'Culture Head' is the Ruler who has the most influence over their own Culture.
The Cultural Head can influence which Innovation should be prioritized for discovery by setting the Cultural Fascination.
The Culture Head can also Reform parts of their Culture.
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.
Culture Head is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A 'Culture Head' is the Ruler who has the most influence over their own Culture." The game also links it as Cultural Head. 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.