Definition - A sub-division of Celtic that includes Welsh, Breton (Brittainy), and Cornish (from Cornwall).

The Lord's Prayer in Proto-Celtic, Ancient British and Brythonic

Oxford English Dictionary -
The term's first citation is from 1884:
"The other group is represented by the people of Wales and the Bretons+the national name of those speaking these dialects was that of Briton+we take the Welsh form of it, which is Brython, and call this group Brythons and Brythonic."
(Rhys Celtic Britain 3 )

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