Outpost 6 was an outpost constructed on an asteroid by Starfleet in 2160. This outpost was one of seven Earth Outpost Stations located in Sector Z-6 on the Federation side of the Romulan Neutral Zone.
During the Neutral Zone Incursion of 2266, the location of Outpost 6 was labeled on a map of Sector Z-6 that was displayed on the bridge viewscreen of the USS Enterprise. (TOS: "Balance of Terror", production art)
In 2366, the location of Outpost 6 was labeled on a tactical intelligence analysis star chart. That year, the outpost received a warning from Starfleet Command about an alleged Romulan base on Nelvana III. (TNG: "The Defector", okudagram)