Segmentum

In Warhammer 40,000, the Imperium of Man has divided the Milky Way galaxy into five zones known as a Segmentum Majors (plural Segmentae Majoris). Originally designed as a means of fleet and shipping control, the Segmentae are administrative divisions of the Adeptus Terra, with each Segmentum ruled over by a Administratum adept, known as the Master of the Segmentum, while the Imperial Guard and Imperial Navy both have Lord Commanders overseeing their operations in those Segmentae. The Imperial Military machine is based at a Forge World in each Segmentum. Of course, interstellar communication being what it is, many of these Segmentae have a federated system where the individual sectors oversee their planets and report back to Segmentum command.

Segmentum Solar

The Imperium's creamy center, where the Sol System is located. As a result, the Segmentum is one of the most heavily industrialized and militarized, as the High Lords of Terra tend to start freaking out whenever a rebellion occurs in their own back yard.

Planets:

• Sol Systems
  • Terra
  • Mars
• Armageddon
• Necromunda
• Fenris

Segmentum Tempestus

The galactic "south" (meaning south of Terra).

Planets:

• Deliverance
• Inwit
• Krieg
• Tallarn

Segmentum Pacificus

The galactic "west", and one of the least explored regions of the galaxy. Much of it was reconquered by Lord Commander Macharius in the Macharian Crusade. In 999.M41, mere months before the 13th Black Crusade, much of the Segmentum Pacificus went into open revolt against the Imperium in the "Night of a Thousand Rebellions".

Planet:

• Sabbat Worlds
• Tanith
• Nova Terra
• Mortikah VII

Segmentum Obscurus

The galactic "north", which has the misfortune of being the location of the Eye of Terror. As a result, the Segmentum Obscurus has one of the largest Segmentum fleets in the Imperium, in an effort to keep the Chaos Space Marines from breaking out.

Planets:

• Cadia
• Caliban's remains
• Medusa
• Vostroya

Ultima Segmentum

The galactic "east" and largest of the Segmentae, clocking in at nearly a third of the galaxy.

Planets:

• Baal
• Badab
• Chogoris
• T'au
• Ultramar
• Valhalla