An executioner is an individual who administered death sentences ordered by the state or other authority.
Due to his role in having 4,000 colonists put to death, former Governor Adrian Kodos of Tarsus IV entered the history books as "Kodos the Executioner". (TOS episode: "The Conscience of the King"; DSC novel: Drastic Measures)
In 2364 Governor Karnas of Mordan IV told Mark Jameson that his executioners were just as efficient as they were last time Jameson had visited the world, if not more so. (TNG episode: "Too Short a Season")