Game concept
Great Building
The game also links it as Great Buildings.
From the game's encyclopedia
Great Buildings are unique Buildings, and are constructed as a result of Great Projects. One famous example is the Great Walls of China.
Great Buildings are usually very powerful, but their utility is tied to specific conditions.
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.
Great Building is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Great Buildings are unique Buildings, and are constructed as a result of Great Projects." It is part of Building. It covers Ruined Building and Rebuilt Building. The game also links it as Great Buildings. 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.