mythical “lost” land supposed once to have connected Cornwall in the west of England with the Scilly Isles lying in the English Channel. The name Lyonnesse first appeared in Sir Thomas Malory's late 15th-century prose account of the rise and fall of King Arthur, Le Morte Darthur, in which it was the native land of the hero Tristan. Arthurian legend, however, had long associated Tristan with Leonois—probably the region around Saint-Pol-de-Léon in Brittany—and this form is the source of Malory's Lyonnesse.

Quite separate from Arthurian legend was a tradition (known at least since the 13th century) that concerned a submerged forest in this region, and a 15th-century Latin prose work, an account of the journeys of William of Worcester, makes detailed reference to a submerged land extending from St. Michael's Mount to the Scilly Isles. William Camden's Britannia (1586) called this land Lyonnesse, taking the name from a manuscript by the Cornish antiquary Richard Carew.