King Arthur was a legendary 6th century British king. He was the central character at the heart of the Arthurian legends that were steeped in chivalry and romance.
In the 19th century, Samuel Clemens wrote the novel A Connecticut Yankee in King Arthur's Court. In the story, a man from the 19th century was transported back to the 6th century. He changed the timeline by introducing gadgets and weapons in the name of progress. (TNG: "Time's Arrow, Part II", "Spirit Folk")
At least two Federation starships were named USS Excalibur after the sword that Arthur pulled out of a stone, thus proving his lineage and with it his rightful claim to the throne of England.
In 2372, Kira Nerys and Jadzia Dax visited a holosuite program based on King Arthur's Court at Camelot on at least two occasions. (DS9: "The Way of the Warrior", "The Muse")