In Warhammer 40,000, the Imperium of Man has divided the Milky Way galaxy into five zones known as a Segmentum Majors (plural Segmenta 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 Segmenta. The Imperial Military machine is based at a Forge World in each Segmentum. Of course, interstellar communication being what it is, many of these Segmenta have a federated system where the individual sectors oversee their planets and report back to Segmentum command.
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.
It is approximately a 44,000 light year diameter circle centered on Sol System, and is approximately 1.52 billion square light years.
The galactic "south" (meaning south of Terra). This segmentum is generally safe from the antics of Chaos due to being far from the Eye of Terror, instead having to put up with Xenos invasions and raids, a notable example being Hive Fleet Leviathan.
The galactic "west", and one of the least-explored regions of the galaxy. This part of the Imperium has regularly rebelled, starting with the Nova Terra Interregnum. There was also the smaller Sabbat Worlds Crusade, a major Chaos incursion that took lots of resources to quell. Much of the Segmentum 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". It is also home to the Q'Orl, one of the few Xenos empires that actually stands a chance of taking the Imperium head-on should they mobilize en masse. By stand a chance we mean that they might actually survive long enough for the Imperial troops to be redirected to some other front.
It is approximately a quarter-torus 11,000 light years wide radially and with outer and inner arcs 52,000 light years and 35,000 light years long, respectively; it comprises roughly 475,000 square light years, which is ridiculously tiny on a galactic scale (proving once again that nobody at GW has anything resembling a sense of scale).
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. A large section of the Segmentum is "obscured" by the Eye of Terror, blocking the light of the Astronomicon, meaning navigation and communication do not function well (if at all) on that edge of the galaxy. The Segmentum is now isolated from the rest of the Imperium due to the Great Rift and came to be part of the Dark Imperium being beset by the Iron Warriors.
The galactic "east" and largest of the Segmenta, clocking in at nearly a third of the galaxy. Includes the galactic core and the realm of Ultramar. The Segmentum marks the rough outer limits of the Astronomican's influence, making any travel further treacherous for any Imperial. It is also bordered by a number of physical obstacles, such as the Maelstrom, the Tau Empire, the Ork Empire of Charadon and the Ork Empire of Octarius. Part of the Segmentum now lies beyond the Great Rift.