Federation-Tzenkethi border

From Memory Alpha, the free Star Trek reference.

The Federation-Tzenkethi border was the division between the United Federation of Planets and the Tzenkethi. The Federation had several colonies near it in the early 2370s, such as Barisa Prime. The Helaspont Nebula is also located nearby.

In late 2371, a Founder took control of the USS Defiant and had the starship cross the border into Tzenkethi space. (DS9: "The Adversary")