The Delta Triangle was a vast uninhabited sector of the Milky Way Galaxy, located three weeks away from the nearest Federation starbase via subspace radio. An unusal feature of the Delta Triangle was its ability to cause starship sensors to fluctuate chaotically while within the region. This effect was caused by the parallel time continuum known as Elysia. (TAS: "The Time Trap")
Several decades prior to the 2260s, the starship Bonaventure explored the Triangle but became trapped within it. Species whose ships had become trapped over the years included Andorian, Tellarite, Klingon, Orion, Vulcan, Romulan, Human, Phylosian, and Gorn.
In 2269, the USS Enterprise surveyed the region to investigate and possibly determine the cause of the high number of mysterious disappearances of starships that have been recorded there since the distant past.