Game concept
Usurp Title
The game also links it as Usurp, Usurpation, Usurped.
From the game's encyclopedia
A Title held by a foreign Ruler can usually be Usurped if more than 50% of its De Jure Counties are within your Realm. This process gives the Title to you, which is particularly important for ensuring you are the Rightful Liege of your Vassals.
Usurpation is more difficult if the current holder's Faith is hostile to yours; in this case usurpation is only possible if the holder no longer possesses any of its De Jure Counties inside his Realm.
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.
Usurp Title is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Title held by a foreign Ruler can usually be Usurped if more than 50% of its De Jure Counties are within your Realm." The game also links it as Usurp, Usurpation and Usurped. 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.