Game concept
Casus Belli
The game also links it as Casus Bellis.
From the game's encyclopedia
A Casus Belli ('Cause for War') required to declare War on another Ruler.
The Casus Belli specifies what the War is about, usually a specific territory — the War Target. It is not possible to gain anything more than the Casus Belli allows, regardless of how soundly the enemy is defeated.
One of the most important Casus Bellis are Claims, whether they come from Succession or the Fabricate Claim on County Task.
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.
Casus Belli is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Casus Belli ('Cause for War') required to declare War on another Ruler." It covers Mongol Invasion Casus Belli, Migrate to Pannonia Casus Belli, Imperial Expedition, Ducal Conquest Casus Belli, Forced Vassalization Casus Belli and 9 more. The game also links it as Casus Bellis. 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.