Game concept
Title Heir
The game also links it as Title Heirs.
From the game's encyclopedia
The Title Heir is the Heir to a specific Title with its own Title Succession. They will receive that Title when the current Holder dies.
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.
Title Heir is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The Title Heir is the Heir to a specific Title with its own Title Succession." It is part of Heir. The game also links it as Title Heirs. 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.