The Mempa sector was an area of space inside the Klingon Empire, on the Federation border. This sector contained the Mempa system and the star Beta Thoridar.
The Mempa sector was a main base of the House of Duras in 2367, where they gathered their fleet to challenge Gowron's forces at the start of the Klingon Civil War. (TNG episode: "Redemption, Part I")
Although Gowron was able to destroy their Mempa sector bases, Duras was able to drive Gowron out of the Mempa sector, to Beta Lankal. (TNG episode: "Redemption, Part II")