The Federation-Tzenkethi border was the line of demarcation between the United Federation of Planets and the Tzenkethi Coalition.
The Helaspont Nebula was located not far from the border. (Star Trek: Deep Space Nine: "The Adversary")
The Hinterlands was an area near the border where Cardassian, Federation and Tzenkethi space intersected. (Star Trek: The Prospect Chronicles: "The Tzenkethi Incident")
In 2365, several Starfleet starships, including the USS Des Plaines, disappeared along the Federation-Tzenkethi border. Starfleet Command sent the Special Operations starship USS Prospect to investigate and possibly secretly cross the border to infiltrate Tzenkethi space to find the whereabouts of the missing ships. (Star Trek: The Prospect Chronicles: "The Tzenkethi Incident")