Game concept
Spouse
The game also links it as Spouses, Spouse's.
From the game's encyclopedia
A Spouse is a Character's husband or wife; the result of a Marriage.
In order to have legitimate offspring and produce Heirs, you should have a Spouse or Concubine (though Bastards can be legitimized).
The only way of forging an Alliance is through marriage or Betrothal; your own or that of a close relative.
Your (main) Spouse is also a member of your Council and can perform many useful Councillor Tasks.
Faith Doctrines determine how many Spouses you can have and how they can be Divorced.
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.
Spouse is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Spouse is a Character's husband or wife; the result of a Marriage." It covers Primary Spouse and Available Spouse. The game also links it as Spouses and Spouse's. 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.