Sector 78 was a region of Federation space, a sector located near the Klingon Empire in the galaxy's Alpha or Beta Quadrants, and was a location on star charts used by the Federation in the 23rd century.
History[]
In 2270, the starship USS Enterprise was assigned to investigate an armed, automated vessel that had been reported in Sector 78, as Starfleet suspected that it might be a Klingon incursion. On stardate 6013.4, the Enterprise was in the sector, near or in orbit of the planet N'cai.
Three months later, a temporal disruption threw the Enterprise back to stardate 6013.4, but to a different location. Starbase 7 checked to see if there were now two Enterprises, but scanners showed that the Enterprise in Sector 78 had disappeared. (TOS - Mudd's Angels short story: "The Business, As Usual, During Altercations")