Game concept
Powerful Vassal
The game also links it as Powerful Vassals, Powerful, Powerful Vassal's.
From the game's encyclopedia
'Powerful Vassals' are the handful of Vassals with the most Soldiers and highest Income. You need to pay careful attention to these vassals, because they expect to hold a seat on your Council and will have significantly lower Opinion of you if they do not.
Changing a Succession Law requires all Powerful Vassals to either have a positive Opinion of you, be Terrified by you, or be Imprisoned.
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.
Powerful Vassal is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "'Powerful Vassals' are the handful of Vassals with the most Soldiers and highest Income." It is part of Vassal. The game also links it as Powerful Vassals, Powerful and Powerful Vassal'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.