A king is the monarch of a kingdom.
Data played the role of King Henry V in a reinactment of the eponymous play by William Shakespeare on the holodeck. (TNG: "The Defector")
Leonidas was a Spartan king who led his people in the Battle of Thermopylae. (DS9: "What You Leave Behind")
King Arthur was a mythical English king. (DS9: "The Muse")
Brian Boru was the High King of Ireland in the 11th century. The King of Leinster was also a King in Ireland. (DS9: "Bar Association")
Quark was known as "The Synthehol King" on Ferenginar. (DS9: "Body Parts")
The Doctor once encountered King Hrothgar in a holonovel. (VOY: "Heroes and Demons")
In 2374, Jack, Lauren and Patrick compared Dukat to a king who lost his throne. (DS9: "Statistical Probabilities")
After exposing the Balancar Agricultural Consortium squill stockpiling, Quark called himself "the king of squill". (DS9: "The Magnificent Ferengi")