Game concept
Holder
The game also links it as Holders, Holder's, Holders', Hold, Held.
From the game's encyclopedia
A Title's Holder is the character who owns the title. Barony and County titles will always have Holders once settled, whereas Duchy, Kingdom and Empire titles might not have been created yet. As a further caveat, the Holder of a high-level Title might not actually control any territory within the Title's De Jure area.
Holding higher-tier Titles allows you to have higher-tier Vassals. For example, Dukes can have Counts as Vassals, and Kings can have Dukes.
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.
Holder is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Title's Holder is the character who owns the title." The game also links it as Holders, Holder's, Holders', Hold and 1 more. 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.