Game concept
Heir
The game also links it as Heirs, Heir's.
From the game's encyclopedia
An Heir is a Character that stands to inherit one or more Titles.
Every held Title has an Heir, determined by the Succession Laws of its current Holder. When the current Holder of the Title dies, or otherwise triggers a Succession, the Title's current Heir will inherit it.
Note that Titles held by the same Ruler can have different Heirs. Most notably, the Primary Title is inherited by the Primary Heir and players will keep playing as their Player Heir.
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.
Heir is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "An Heir is a Character that stands to inherit one or more Titles." It covers Primary Heir, Player Heir, Title Heir and Realm Heir. The game also links it as Heirs and Heir's. 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.