An executioner was an individual who administered the death penalty.
Governor Kodos was dubbed "Kodos the Executioner" after ordering the deaths of 4000 residents of the Earth colony on Tarsus IV. (TOS: "The Conscience of the King")
In 2364, Karnas reminded Mark Jameson of the efficiency of his executioners, which the latter had seen before. (TNG: "Too Short a Season")
In 2370, when about to kill The Albino, Kang told him to look upon his executioners. (DS9: "Blood Oath")
In 2372, young Kar told Chakotay that he was not his son, but his executioner. (VOY: "Initiations")
Also that year, Kathryn Janeway told Mabus that the USS Voyager would not survive by making deals with executioners. (VOY: "Alliances")