A legendary Welsh prophet and magician, child of a wizard and a princess, who lived in the 5th century, and was subsequently a prominent personage at King Arthur's' court; prophecies attributed to him existed as far back as the 14th century; Tennyson represents him as bewitched by Vivian; legend also tells of a Clydesdale Merlin of the 6th century; his prophecies, published in 1615, include the former; both legends are based on Armorican materials.