Game concept
Sin
The game also links it as Sins, Sinful.
From the game's encyclopedia
Every Rite considers certain Traits to be Sinful. Characters with these Traits incur Opinion penalties from all characters who consider those traits Sinful, even if they do not share the same Rite. In addition, Characters who have Traits that their own Rite considers Sinful will lose Piety every month.
Exactly what traits are considered to be Sins varies from Rite to Rite, but is typically determined by a combination of that Rite's Tenets and the Religion the Rite belongs to.
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.
Sin is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Every Rite considers certain Traits to be Sinful." It covers Sin Weight. The game also links it as Sins and Sinful. 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.