Britain
wiktionary
From Middle English Breteyn, from Old English Breoton, Bryten etc., from Latin Britannia; later reinforced by Anglo-Norman Britaine, Old French Bretaigne, from Latin Brittannia, variant of Britannia, from Britannī. Ultimately from Proto-Brythonic *Prɨdėn(“Britain”) from *Pritanī (also compare *Prɨdɨn(“Picts”) from *Pritenī), attested to in Ancient Greek as Πρεττανική(Prettanikḗ), compare Welsh Prydain. Doublet of Brittany. More at Britto.
This etymology is incomplete. You can help Wiktionary by elaborating on the origins of this term.
From Latin Britannus (adjective and noun, plural Britannī), apparently from Brythonic (compare Old Welsh Priten).
etymonline
Britain (n.)
proper name of the island containing England, Scotland, and Wales, c. 1300, Breteyne, from Old French Bretaigne, from Latin Britannia, earlier Brittania, from Brittani "the Britons" (see Briton). The Old English place-name Brytenlond meant "Wales." If there was a Celtic name for the island, it has not been recorded.
