Game concept
Legitimate
From the game's encyclopedia
Children born of Married parents are considered Legitimate, as opposed to Bastards who may not inherit any Titles. The children of Concubines are also considered Legitimate, though of slightly lower status.
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.
Legitimate is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Children born of Married parents are considered Legitimate, as opposed to Bastards who may not inherit any Titles." 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.