Game concept
Siege
The game also links it as Besiege, Besieged, Sieges, Besieging.
From the game's encyclopedia
Siege is when an Army surrounds a Fortified Holding in order to starve the citizens into submission so that it can be Occupied. A Siege starts automatically once an Army enters the location of a hostile Fortified Holding and stands still — if the Army outnumbers the Garrison.
The besieging Army can Assault the Holding if its walls are Breached.
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.
Siege is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Siege is when an Army surrounds a Fortified Holding in order to starve the citizens into submission so that it can be Occupied." The game also links it as Besiege, Besieged, Sieges and Besieging. 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.