Game concept
Martial Skill
The game also links it as Martial.
From the game's encyclopedia
Martial Skill indicates how good a Character is at waging War and leading Armies in Battle.
A high Martial Skill is important for Army Commanders, since it improves their Advantage in Battle.
The corresponding Lifestyle is the Martial 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.
Martial Skill is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Martial Skill indicates how good a Character is at waging War and leading Armies in Battle." It is part of Skills. The game also links it as Martial. 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.