Game concept
Hostage
The game also links it as Hostages, Hostage's.
From the game's encyclopedia
A Close Family Member of a Ruler, sent away from their Home Court as an assurance of peace or submission.
The reputation of Rulers who declare War despite sending or receiving a Hostage will be dramatically damaged, and Wardens may freely execute the Hostages of their enemies.
Well-treated Hostages are likely to become loyal to their Warden, who may gain a Hook, or even become Friends with them.
Hostages must be landless children, siblings, nieces, nephews, or grandchildren of their Home Court's Ruler.
Wardens have a chance to gain a Perpetual Hook on a child Hostage when they come of age, based on personality, opinion, and years spent at the Wardens court.
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.
Hostage is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Close Family Member of a Ruler, sent away from their Home Court as an assurance of peace or submission." The game also links it as Hostages and Hostage's. Patch 1.20.1.2, from game/common/game_concepts/05_bp2_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/05_bp2_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.