The Triangulum system was a star system.
In 2366, Captain Jean-Luc Picard recalled a Starfleet admiral who had had difficulty finding some renegade Andorians in the Triangulum system. This was due to the Andorians having dismantled their ship in order to hide it. (TNG: "The Survivors")
The Triangulum seems to have been named after the Triangulum constelation, and might be located in that constelation.