What is "logres"

Logres (also Logris or Loegria) is the name of King Arthur's realm in the Matter of Britain. It derives from the Welsh word Lloegr, a name of uncertain origin meaning " England".

In Arthurian contexts, "Logres" is often used to describe the Brittonic territory roughly corresponding to the borders of England before the area was taken by the Anglo-Saxons. According to Geoffrey of Monmouth's influential pseudohistory Historia Regum Britanniae, the realm was named after the legendary king Locrinus, the oldest son of Brutus of Troy. In his Historia, Geoffrey uses the word "Loegria" to describe a province containing most of England excluding Cornwall and possibly Northumberland, as in this example from section iv.20 (from the Penguin Classics translation by Lewis Thorpe):

The name "Logres" is used in many works of modern fantasy set in Britain, for example, C. S. Lewis's That Hideous Strength and Susan Cooper's Over Sea, Under Stone.