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.
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.
After nearly falling victim to a mouse trap set by the Klingons, the Enterprise along with the Klingon battle cruiser IKS Klothos, became trapped in a pocket time continuum of the Triangle known as Elysia. (TAS: "The Time Trap")