Triangulum system
The Triangulum system was a star system that was located somewhere in the Alpha or Beta Quadrant.
In 2366, after Worf declared that he could not find any trace of those responsible for decimating the Rana IV colony, Captain Jean-Luc Picard recalled a Starfleet admiral who had had difficulty locating some renegade Andorians in the Triangulum system. It turned out that the Andorians dismantled their ship in order to hide it. (TNG: "The Survivors")
This system was only mentioned in dialogue.