Logres

The word comes from the Welsh "Lloegyr" or "Lloegr" and is used in such Welsh writings as "The Dream of Rhonabwy" and various of the "Welsh Triads," or "Trioedd Ynys Prydein."

Chretien is the next (chronologically) author generally noticed for using the term in books such as "Lancelot" and "Percival." Because of other poetic license in his writings, it seems that scholars tend to take the term more as a poetic name than as a clear cut geographical or political region. The Vulgate Cycle also uses the term, and is generally regarded as a more viable source of historic information than Chretien's works.

From there the word gets Latinized into "Loegria" and this change is generally credited to Geoffrey of Monmouth, but it's debatable as to whether this was a merely philological change, or whether it was an attempt to tie the word more closely to "Logrine" or "Locrine" - the name of the mythical king Brute's eldest son. Milton seems to indicate the latter in book one of his "History of England", as follows:

“His three sons divide the land by consent; Locrine had the middle part, Loëgra ...”

From there it seems a short jump to "Logres;" indeed, I have seen Chretien quotes using the term "Logres" as well as quotes using "Loegria."

I ran across this term reading That Hideous Strength by C.S. Lewis. It's the third book of his Space Trilogy, in which he makes vague allusions to all kinds of Arthurian legends, including a spectacular cameo of Merlin.

Interestingly, to me anyway, two of the other members of Lewis' literary group "The Inklings" also make allusions to Logres in their books and poems: J.R.R. Tolkien, and Charles Williams. (The group also included Owen Barfield, and probably other cool authors of which I'm unaware.) Charles Williams entitled one of his books of poetry "Taliessin Through Logres."

There's also some legend out there about the "Sword of Logres" which is one of the four "Swords of the Islands." ...I'm wondering if the sword was related to Excalibur. If one believes everything one reads, it seems plausible... Sir Meliot of Logres was supposedly cousin to the Lady of the Lake, and ...but now I've gone past the realm wondering and into the realm of wandering... to tales beyond my ken...