The Orion Arm, or simply the Arm, was a region of space closely related to the Orion constellation.

In FASA RPG sources, the term "Orion Arm" is used in a context that implies a much smaller region of space than the Orion Arm of the Milky Way Galaxy known to astronomy. Given the name, description, and inclusion of Rigel, this is likely to have been mistaken for the Orion constellation. The description matches that of Rigel, Orion's Belt, and the Flame Nebula.

The Orion Arm consisted of a belt of ionized hydrogen gas clouds and a number of large, hot, bright stars. One of these stars was Rigel, which lay just on the Coreward side of the Arm, and was in equal reach of both sides. At Rigel, the Arm lay equidistant between the United Federation of Planets and the Klingon Empire, and was home to the Orion Colonies and various other alien races (FASA RPG modules: The Orions: Book of Common Knowledge, Star Trek IV Sourcebook Update).

In ancient times, prior to 1,498,000 BCE (reference stardate -15,000/00), the Masters of Rigel ruled an empire across the Orion Arm. Much later, the Orion War was waged across the Arm from 94–57 BCE (stardates −20/94 to −20/57).

The area was poor in resources (FASA RPG module: The Orions: Book of Common Knowledge) and considered something of a "dead zone" and a natural barrier to colonial expansion. Early in their conflicts with the Klingon Empire in the early 23rd century, the Federation was willing to concede the area of the Orion Arm to the Klingons. However, the Klingons had little interest in it while Federation colonial and trade expansion towards and into the Arm was unstoppable (FASA RPG module: Star Trek IV Sourcebook Update).

Later, following the Four Years War between the two powers of stardates 1/94–1/98 (c. 2250s), the Federation's Rigel Demilitarized Zone Commission considered designating the entire Orion Arm a demilitarized zone under the authority of the Orion Colonies. The idea was abandoned in favor of the Orion Neutrality Area, which allowed the Federation to annex over 80% of the Arm and the Colonies within (FASA RPG module: The Orions: Book of Common Knowledge).

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