Game concept
Treasury Budget Deficit
The game also links it as Deficit.
From the game's encyclopedia
When spending more than the Budget Capacity, an amount will be subtracted from the monthly Treasury income.
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.
Treasury Budget Deficit is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "When spending more than the Budget Capacity, an amount will be subtracted from the monthly Treasury income." It is part of Treasury Budgeting. The game also links it as Deficit. Patch 1.20.1.2, from game/common/game_concepts/tgp_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/tgp_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.