Game concept
Empire
The game also links it as Empires.
From the game's encyclopedia
An Empire is a Title one Tier below a Hegemony and one above a Kingdom.
The Holder of an Empire has De Jure rights to its constituent territory and can have Kings as Vassals.
Empires can be dormant and have no current Holder. Such Titles can be created by a Ruler who controls more than 80% of the Counties within.
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.
Empire is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "An Empire is a Title one Tier below a Hegemony and one above a Kingdom." It is part of Title. The game also links it as Empires. 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.