Game concept
Army Commander
The game also links it as Commander, Commanders, Army Commanders, Commanding.
From the game's encyclopedia
An Army Commander is a Character in command of an Army. Any of your Courtiers or direct Vassals can potentially be assigned as a Commander (subject to age, Health, and Doctrines).
In Battle, the Martial Skill of the Commander heavily influences the Advantage. As long as there is a Commander present, additional Rolls will be made in attempt to increase the Advantage.
In addition, Commander Traits can provide an Army with many different perks, both in and out of Battle.
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.
Army Commander is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "An Army Commander is a Character in command of an Army." The game also links it as Commander, Commanders, Army Commanders and Commanding. 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.