Game concept
Betrothal
The game also links it as Betrothed, Betrothals.
From the game's encyclopedia
A Betrothal is a promise of a future Marriage. It can be made when either party is not yet an Adult, or when a Grand Wedding is Promised.
Betrothals can grant Alliances just like Marriages. Regular Betrothals can last as long as needed and be easily broken. They can be converted into a proper Marriage upon adulthood, while Grand Wedding Betrothals are converted into a Marriage by completing a Grand Wedding.
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.
Betrothal is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Betrothal is a promise of a future Marriage." The game also links it as Betrothed and Betrothals. 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.