Game concept
Realm Heir
The game also links it as Realm Heirs.
From the game's encyclopedia
A Realm Heir is one of the Heirs of a Realm, and will receive either all or some of the Titles held by the Ruler of that Realm when that Ruler dies.
A Realm may have one or more Realm Heirs depending on its 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.
Realm Heir is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Realm Heir is one of the Heirs of a Realm, and will receive either all or some of the Titles held by the Ruler of that Realm when that Ruler dies." It is part of Heir. The game also links it as Realm 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.