Game concept
Regiment
The game also links it as Regiments.
From the game's encyclopedia
A Regiment is a group of Soldiers of a single type (e.g., archers, footmen, or Levies). Although Levies are organized into Regiments, the ones of real interest are the Men-at-Arms.
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.
Regiment is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Regiment is a group of Soldiers of a single type (e.g., archers, footmen, or Levies)." It covers Stationed Regiment and Soldiers. The game also links it as Regiments. 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.