Game concept
Prowess Skill
The game also links it as Prowess, Natural Prowess.
From the game's encyclopedia
Prowess is a Skill that reflects the Character's aptitude in personal combat.
A high Prowess Skill means they are more likely to survive and perform well both as Knights in Battles and in Duels.
Each point of Prowess gives a Knight:
• Damage: 50
• Toughness: 10
Increasing prowess through natural sources (not through Artifacts) will increase a Character's muscle mass.
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.
Prowess Skill is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Prowess is a Skill that reflects the Character's aptitude in personal combat." It is part of Skills. The game also links it as Prowess and Natural Prowess. 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.