The Triangulum system was a star system.
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.