Game concept
Diplomacy Skill
The game also links it as Diplomacy.
From the game's encyclopedia
Diplomacy is a Skill representing a Character's ability to charm, negotiate and persuade.
Diplomacy directly affects the Opinions other characters have of you. It is also used to further Diplomatic Schemes.
The corresponding Lifestyle is the Diplomacy Lifestyle.
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.
Diplomacy Skill is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Diplomacy is a Skill representing a Character's ability to charm, negotiate and persuade." It is part of Skills. The game also links it as Diplomacy. 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.