Melkotian space was a territorial region of space claimed by the Melkotians. It was located somewhere in the Alpha or Beta Quadrant. Ships entering into this space were confronted by a warning buoy which warned the ship that it was encroaching upon the "space of the Melkot" and that they should turn around immediately or face the consequences.
In 2268, the USS Enterprise became the first Federation starship to be given permission to enter the space. The starship then proceeded to the Melkot's homeworld. (TOS: "Spectre of the Gun")
In the revised Stellar Cartography: The Starfleet Reference Library (Beta Quadrant: Local Space Extract), there was a star system named Melkotia. This system might be located in this region and have an association with the Melkotian homeworld.