Game concept
Attrition
From the game's encyclopedia
Armies can suffer from Attrition, which is when Soldiers are lost due to desertion, disease or starvation.
Attrition occurs when an Army is out of Supplies, besieging a Fortified Holding, or moving from one hostile County into another if the target location doesn't border a County already under control.
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.
Attrition is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Armies can suffer from Attrition, which is when Soldiers are lost due to desertion, disease or starvation." 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.