Game concept
Ruler
The game also links it as Rulers, Ruler's.
From the game's encyclopedia
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.
Ruler is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Ruler is a Character who holds at least one Title." It covers Vassal, Secular Ruler, Liege, Independent Ruler, Overlord and 2 more. The game also links it as Rulers and Ruler's. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
for a guide, post or blog
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.