Game concept
Holy Order
The game also links it as Holy Orders, Holy Order's, Holy Orders'.
From the game's encyclopedia
A Holy Order is an organization of religious devotees which exists to serve the Faith.
An Order will lease Holdings from other Rulers. They may have a Holy Order Patron who controls their Capital, and to whom they provide benefices.
There are two types of Holy Orders. Monastic Orders serve to increase Development, spread Tenets, and provide Spiritual Fulfillment. Military Orders may be hired into Holy Wars against a hostile or evil.
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.
Holy Order is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Holy Order is an organization of religious devotees which exists to serve the Faith." The game also links it as Holy Orders, Holy Order's and Holy Orders'. 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.