Game concept
Mercenary Company
The game also links it as Mercenary, Mercenaries.
From the game's encyclopedia
Mercenaries are Soldiers for hire who fight in exchange for Gold. Mercenary Armies come in various sizes, with different types of Men-at-Arms and a number of Knights.
Mercenaries sign up for 36 month contracts that can be extended for another 36 months if you are still at War. Mercenary contracts persist even if their Regiments are dismissed — they can be raised again without further cost.
You are allowed to go into Debt in order to hire Mercenaries (up to 24 months).
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.
Mercenary Company is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Mercenaries are Soldiers for hire who fight in exchange for Gold." The game also links it as Mercenary and Mercenaries. 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.