The Altair system was an inhabited planetary system. The system was located in Sector 9. Primary's name was Altair. Planets in this system included Altair III, Altair IV, and Altair VI. This system was of strategic importance to the Federation in the mid-23rd century. (TOS: "Amok Time", "Wolf in the Fold", TNG: "Encounter at Farpoint") The Altair system was located in the Alpha Quadrant. Its location in the Milky Way Galaxy was depicted on a Federation star chart in 2293. (Star Trek VI: The Undiscovered Country, production art) In the 2280s, a freighter could travel in nineteen periods from Altair to reach Gamma Hydra sector, Section 10. (Star Trek II: The Wrath of Khan)
Planets of the Altair system were the origin of such consumables as Altair water, the champagne Altairian Grand Premier, and the Altair sandwich. (Star Trek III: The Search for Spock; TNG: "Datalore"; DS9: "You Are Cordially Invited").
The Altair system had been involved in long interplanetary wars in the early 2260s. The Federation attempted to help stabilize this by supporting the inauguration of a new president on Altair VI in 2267. Three starships, including the USS Enterprise, were to rendezvous here for this historic occasion. Their presence was a demonstration of friendship and strength which was thought to cause ripples all the way to the Klingon Empire. (TOS: "Amok Time")
In the early 2360s, first officer William Riker refused Captain Robert DeSoto of the starship USS Hood to beam down to Altair III. (TNG: "Encounter at Farpoint") Captains Rixx and Jean-Luc Picard had initially met at the Altairian Conference sometime prior to 2364. (TNG: "Conspiracy")
"The Explored Galaxy"
"The Explored Galaxy" was a map of charted space in the Alpha Quadrant. The Altair system, represented here by Altair VI, 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 the Alpha 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".
Altair (Alpha Aquilae): The system was located in the Alpha Quadrant. This was a single star system. Altair was an A-class star with a magnitude of +3, which was ten times brighter than Sol. In the mid-22nd century, this system, located in non-aligned space, was a destination on the Earth and Vulcan trade routes. In 2378, this system was located in or near Federation space. (pg. 44, "United Federation of Planets I")