Game concept
Lover
The game also links it as Lover's, Lovers, Lovers'.
From the game's encyclopedia
A Lover is a Special Relation indicating an ongoing amorous and sexual association between two Characters.
As they often take place outside the bonds of Marriage or Concubinage, most Lover Special Relations begin as Secrets.
If a man and a woman are Lovers, the woman may become pregnant with the man's child. If they are not Married (and one is not the Concubine of the other), the child will be a Bastard, secret or otherwise.
Lovers who are deeply loyal and affectionate might become Soulmates.
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.
Lover is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Lover is a Special Relation indicating an ongoing amorous and sexual association between two Characters." It is part of Special Relation. The game also links it as Lover's, Lovers and Lovers'. 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.