(covers information from several alternate timelines)
In stellar cartography, a sector, which was referred to as a star sector or space sector, was a gridded region within the Milky Way Galaxy. Sectors were composed of an area and volume encompassing several light years and typically contained several star systems. A group of sectors was called a sector block, which was located in a quadrant. The sector system and their numbering was used by United Earth, Vulcans, and other spacefaring races at least by the 22nd century. In 2369, the extragalactic space was not divided into sectors, and ships that were sent on deep space cartography missions were listed as being in extragalactic space. (TNG: "Aquiel"; ENT: "Broken Bow", "Detained"; Star Trek Into Darkness)
Size and range
William T. Riker felt that the warp coil was perhaps one of the greatest advances in Humanity since the 22nd century, explaining that "before there was warp drive, Humans were confined to one sector of the galaxy." (TNG: "A Matter of Time")
When the USS Enterprise was probed while en route through an unfamiliar sector of space in 2269, the crew was able to determine the source as being from a planet in the Taurean system, a system located twenty light years away, at the extreme edge of their sector. (TAS: "The Lorelei Signal")
While en route to the planet Barisa Prime in 2371, Captain Benjamin Sisko noted that the closest ship in that sector, the USS Ulysses, which was exploring the Helaspont Nebula, was located twenty hours away at maximum warp. (DS9: "The Adversary")
The territory occupied by the Swarm species covered "a huge area of space" and included "hundreds of sectors." For the USS Voyager to go around that region, it would have taken over fifteen months at maximum warp. (VOY: "The Swarm")
Voyager's use of Tash's catapult "hurtled" the ship across thirty sectors of space in less than an hour, a journey that would have otherwise taken three years to accomplish. (VOY: "The Voyager Conspiracy")
Sector Z-6 encompassed both Federation and Romulan space, and during the 23rd century, included seven Earth Outpost Stations, the Romulan Neutral Zone, and Romulus and Romii. (TOS: "Balance of Terror")
The sector in which the Enterprise-D intercepted the class 8 probe occupied by Ambassador K'Ehleyr was said to contain very little, yet included four colonies and nine outposts throughout. A nearby sector that was threatened by the Klingon sleeper ship IKS T'Ong contained thirteen Federation colonies. (TNG: "The Emissary")
When the shuttlecraft Cochrane achieved warp 10, it collected data describing "literally every cubic centimeter" of the sector they were in – an amount of information exceeding five billion gigaquads. (VOY: "Threshold")
Most sectors were numbered, and in some cases, named after an important star system in that sector. In Federation nomenclature, the Sol system and Earth were located in Sector 001. (TNG: "The Best of Both Worlds") Others were named after a key star system, such as the Mutara sector. (Star Trek II: The Wrath of Khan) In other cases, they were given a designation consisting of both a name and number, such as Rhomboid Dronegar sector 006. (TNG: "Samaritan Snare")
A subsector was a region of space smaller than a sector and located within sectors. Subsectors were also numbered, for example Subsectors 4534-4432 within sector 450/32450 (TNG: "Conspiracy" display graphic) and subsector ref 321-053 within sector 30-489.
List of sectors
The Star Trek Encyclopedia (3rd ed., p. 434) defined a sector as,
"In interstellar mapping, a volume of space approximately twenty light-years across. A typical sector in Federation space will contain about 6 to 10 star systems, although sectors toward the galactic core will often contain many more. The Milky Way Galaxy is divided into hundreds of thousands of sectors, grouped into four quadrants. Sectors are usually numbered, although in common usage they are often named for a major star or planet located in that sector."
The Star Trek Encyclopedia (3rd ed., p. 434) authors additionally said,
"The numbering system for sectors had been inconsistently used (and sometimes interchanged with quadrants) during the show, especially in its early days. We assume that some sectors may retain older designations from previous mapping systems, much as present-day astronomers still use NGC and Messier catalog numbers."