Painting of Viking explorer Leif Ericson
The Vikings (Old Norse: vikingr) were a group of early seafaring Humans from Earth's Scandinavia region, including the future nation of Denmark. (VOY: "Heroes and Demons")
In the late 10th century, Leif Ericson was a Viking explorer. (TOS: "Space Seed")
In 1014, a band of Vikings attacked an Irish monastery in the Battle of Clontarf. (DS9: "Bar Association")
In 1953, when Herbert Rossoff and Douglas Pabst almost got into a fight, Julius Eaton called for calm and remarked "we're writers, not Vikings." (DS9: "Far Beyond the Stars")
In 2372, Miles O'Brien and Julian Bashir participated in a holosuite program where they fought against the Vikings at the Battle of Clontarf. (DS9: "Bar Association") Rom described Bashir's Viking program as "stupid." (DS9: "It's Only a Paper Moon")