The Archanis sector was a region of space between the United Federation of Planets and the Klingon Empire. The sector was originally Klingon territory, but was relinquished to the Federation in the mid-23rd century. It contains the planet Archanis IV as well as a number of Federation starbases and outposts. (DS9: "Broken Link")
In late 2372, Chancellor Gowron demanded that the Federation abandon the sector to the Klingons, at the urging of who he believed to be General Martok. In reality this Martok had been replaced by a changeling. Shortly after, a Klingon task force took the sector starting the Federation-Klingon War. (DS9: "Apocalypse Rising")
Background information Edit
According to the Star Trek: Star Charts, on page 62, this sector was named after the Archanis system. This sector was located in the Beta Quadrant. According to Startrek.com, the Archanis sector was adjacent to the Bajor sector.