Game concept
Head of Faith
The game also links it as Heads of Faith, Head of Faith's.
From the game's encyclopedia
A Head of Faith is the formal leader of a Faith. The powers and nature of the Head of Faith depends on the Doctrines of the Faith.
Some Faiths have a Spiritual Head of Faith (e.g. the Pope), some a Temporal Head of Faith (e.g. a Caliph), and some do not have a Head of Faith at all.
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.
Head of Faith is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Head of Faith is the formal leader of a Faith." It is part of Faith. It covers Spiritual Head of Faith, Temporal Head of Faith and Challenger Head of Faith. The game also links it as Heads of Faith and Head of Faith's. 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.