Game concept
Holy Relic
The game also links it as Holy Relics.
From the game's encyclopedia
A Holy Relic is an Artifact that holds religious significance for a Faith. A Holy Relic can be Enshrined in a Holy Site of that faith to gain benefits.
Some artifacts are inherently holy for a faith, but artifacts can also be Sanctified to become a holy relic.
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 Relic is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Holy Relic is an Artifact that holds religious significance for a Faith." It is part of Holy Site. The game also links it as Holy Relics. Patch 1.20.1.2, from game/common/game_concepts/pam_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/pam_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.