The Klingon-Romulan border was the division between the Klingon Empire and the Romulan Star Empire.
Khitomer was located near the border on the Klingon side. (Star Trek VI: The Undiscovered Country)
In early 2367, Jean-Luc Picard placed a fleet of twenty-three Federation starships, designed to establish a tachyon detection grid along the Klingon-Romulan border in an attempt "to prevent any external power from interfering in Klingon affairs." (TNG: "Redemption II")
In early 2372, the Klingon Empire attacked several Romulan outposts along the border. (DS9: "Hippocratic Oath")