Game concept
Faction Military Power
The game also links it as Military Power.
From the game's encyclopedia
Faction Military Power is a measure of the total military strength of a Faction relative to their target.
Each individual faction member adds their full strength to the Faction as long as they remain in the Faction. If the faction target is the Liege of a member, the strength of the member is also subtracted from the relative power of the liege.
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.
Faction Military Power is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Faction Military Power is a measure of the total military strength of a Faction relative to their target." It is part of Faction. The game also links it as Military Power. 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.