Game concept
Secular Ruler
The game also links it as Secular Rulers, Secular.
From the game's encyclopedia
A Secular Ruler is a Ruler that is not part of a religious Government Form.
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.
Secular Ruler is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Secular Ruler is a Ruler that is not part of a religious Government Form." It is part of Ruler. The game also links it as Secular Rulers and Secular. 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.