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 unusual 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.
The Delta Triangle was a region of space known for having a high number of mysterious disappearances of starships recorded since ancient times. In 2269, the USS Enterprise was assigned to a survey mission the area, and possibly determining the cause of these disappearances. In the decades prior, the starship Bonaventure, as well as a number of vessels operated by Andorians, Tellarites, Klingons, Kzinti, Orions, Vulcans, Romulans, Humans, Phylosians, and Gorn became lost in the Delta Triangle.
The Klingons attempted to use the phenomena surrounding the region as a means of springing a mouse trap on the Enterprise, figuring that when the Federation investigated, the loss of the Enterprise would be recorded as just another mysterious starship disappearance. (TAS: "The Time Trap")