The Adeptus Terra, also known as the "Priesthood of Earth," is the central bureaucratic governmental organisation of the Imperium of Man and not really an agency in and of itself.
It is made up of many autonomous departments that receive the orders of the High Lords of Terra, passing them down to the different branches that will enact their commands.
There are only a few organisations in the whole of the Imperium that do not fall under this central command, most notable of which are the Inquisition, the Adeptus Mechanicus and the Adeptus Ministorum, also called the Ecclesiarchy. The Adeptus Astartes -- the Space Marines -- are officially a part of the Adeptus Terra, but retain a high level of autonomy.
The priesthood which serves the Master of Mankind is often referred to as "the right hand of the Emperor." It falls to the Adeptus Terra to interpret the Emperor's will and minister to the Imperium. Many millions of souls labour across the galaxy to serve this vast organisation.
There are numerous ancient institutions that make up the priesthood, each with various names on various planets. These varying "adepta," as they are traditionally known, each have a specific function to carry out in the Emperor's name. Only the Ecclesiarchy and the Imperial Inquisition are not formally a part of the Adeptus Terra.
The Adeptus Terra is an umbrella organisation, and not really an agency in itself, as each of its divisions functions largely autonomously and separately from the others.
The most powerful individual in the Adeptus Terra (and if not in theory, then in practice, the entire Imperium) is the Master of the Administratum, who controls the central, secular bureaucracy of the Imperium.
Imperial Adepta
The following adepta of the Imperial government are all formally part of the Adeptus Terra. The Inquisition, the Adeptus Ministorum (including the Adepta Sororitas) and the Adeptus Mechanicus are all independent of the Adeptus Terra and maintain their own parallel but independent hierarchies.
While the Adeptus Astartes is considered a formal part of the Adeptus Terra, by tradition and out of respect for the Space Marines' genetic connection to the Emperor, its Chapters are granted a wide latitude of political and military command autonomy, though disagreements between some Chapters and the Inquisition and the Adeptus Ministorum have sometimes become so heated they have erupted into armed conflict.
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Sources
- Black Crusade Core Rulebook (RPG), pg. 22
- Codex: Imperialis (2nd Edition)
- Codex: Militarum Tempestus (6th Edition), pp. 5-13
- Dark Imperium (Novel) by Guy Haley, Ch. 9 "Imperator Gloriana," pp. 94-97
- Vaults of Terra: The Carrion Throne (Novel) by Chris Wraight, Ch. 3
- Warhammer 40,000: Rulebook (8th Edition), pp. 24-25
- Warhammer 40,000: Rulebook (Digital Edition) (7th Edition), pp. 275, 278
- Warhammer 40,000 Rulebook (6th Edition), pg. 139