Orion was an inhabited planet. This was the homeworld of the Orions, a warp capable humanoid species. The planet was located in the Orion planetary system. This system was located in the Orion sector, which was a region of space in the Alpha Quadrant. The Orion Syndicate was founded on this world. (Star Trek VI: The Undiscovered Country, production art; TNG: "Conspiracy", display graphic)
During the mid-23rd century, noted Federation archaeologist Doctor Roger Korby translated medical records from the Orion ruins that helped revolutionize modern immunization techniques and were required reading at Starfleet Academy. (TOS: "What Are Little Girls Made Of?")
In 2269, through the assistance of the Guardian of Forever, Starfleet officers Captain James Kirk, Commander Spock and historian Lieutenant Erickson traveled to the dawn of Orion's civilization to view the planet's history unfold firsthand. (TAS: "Yesteryear")
Devna, an Orion trapped in Elysia, told Kirk that she wished to return through the time barrier and see Orion again. Kirk offered to take her back with him, but she declined, stating that she had accepted where she now was in life. (TAS: "The Time Trap")
In 2372, Ferengi entrepreneur Quark attempted to smuggle kemocite on a highly-profitable side trip to Orion under the guise of taking his ship, Quark's Treasure, on a test flight to Earth. This proved to be unsuccessful due to sabotage. (DS9: "Little Green Men")
"The Explored Galaxy"
"The Explored Galaxy" was a map of charted space in the Alpha Quadrant. Orion was located between the Tholian Assembly and Romulus. Both locations were identified in DS9: "The Search, Part II", "Call to Arms" as being located in this quadrant. This chart was also seen in several Star Trek: The Next Generation and Star Trek: Deep Space Nine episodes set in the 24th century, from the year 2364 to 2370. These were: TNG: "Conspiracy", "The Measure Of A Man", "The Emissary", "The Mind's Eye", "The Game" and DS9: "In the Hands of the Prophets", "Cardassians".
Orion was located in the Pi 3 Orionis system. This planet was visited by Zefram Cochrane on his final voyage in 2120. Orion was a non-aligned world before and after 2161, the founding date of the Federation. In 2378, Orion was a destination on the major space lanes. (pgs. 36, 45, 60)