Game concept
Nickname
The game also links it as Epithet, Cognomen, Nicknames, Epithets, Cognomens.
From the game's encyclopedia
Nicknames, Epithets, and Cognomens are all secondary names either taken by or attributed to Characters (e.g. Charles 'the Bald'). Nicknames are purely aesthetic, though the circumstances surrounding their acquisition are often dramatic or noteworthy affairs.
Bad Nicknames will sometimes be overridden by good ones (such as William 'the Bastard' becoming William 'the Conqueror').
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.
Nickname is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Nicknames, Epithets, and Cognomens are all secondary names either taken by or attributed to Characters (e.g." The game also links it as Epithet, Cognomen, Nicknames, Epithets and 1 more. 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.