Game concept
Line of Succession
The game also links it as Line.
From the game's encyclopedia
The Line of Succession is the order in which certain characters stand to inherit certain Titles, determined by the Succession Law.
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.
Line of Succession is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The Line of Succession is the order in which certain characters stand to inherit certain Titles, determined by the Succession Law." The game also links it as Line. 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.