Lyonesse is the Lost Land of English mythology. It is frequently (and mistakenly) referred to as Avalon which is in fact somewhere completely different.

Lyonesse is regarded as the birthplace of Sir Lancelot and situated somewhere of the coast of West Wales. Welsh history and myth name it "The Drowned Hundred", land of bards and artists.

The major difference between Lyonesse and Avalon is that the former is a drowned land, whereas the latter is now merely mundane.