Celtic Britons

The Britons (also called Brythons) were the people who spoke a Celtic language known as Common Brittonic. They lived in Great Britain during the Iron Age, Roman Britain and the Sub-Roman period following the Romans' departure from Britain. Following the arrival of the Anglo-Saxons, who occupied most what is now the country of England, some of the Britons migrated to Wales, Cornwall and southern Scotland, while others moved to Armorica and renamed it Brittany. Those who remained were absorbed into Anglo-Saxon society.
Name.
The name of the Britons in the modern English language is borrowed in part from , and in part from the French language of the Middle Ages (). During Roman Britain, the Latin word and its plural, , took the place of the Romans' earlier word for a person from Britain, , and its plural, the . The people of Roman Britain (the Romano-British population) named themselves in Latin. Other people also named the Britons this way: the work of the Roman historian Procopius names them as and in Ireland, the Britons were named the . Some modern Celtic languages also use this form for Britain was well: and .
The Anglo-Saxon historian Bede, writing in Latin, used the name , with the plural "". The "e" vowel in this spelling could be taken from the Anglo-Saxon language's word for the Britons: , also spelled "Britt-", "Bryt", "Brytt"-, "Bret", and "Brit". The Anglo-Saxons probably took this name from the Romano-British. The was a word used in Britain from the 5th century. Whether the Anglo-Saxons took the name "Brett" from the Latin name or from a British word of the same meaning is unknown. The Anglo-Saxons must have started using the word "Brett" by the 6th century. If they had copied the British word at a later time, the word would have had -"th"- instead of -"t"-, like the . In that case, the Old English copy would have had -"þ"- or -"ð"- in the place of -"tt"-.
The name continued to be used in Middle English, with the spellings "Bret", "Brette", "Brettis". Middle English also used the forms "Brut" and "Brutt"-, with the spelling copied from the name of Brutus of Troy. Mythology in the Middle Ages said that Britain was named after Brutus. In Early Modern English, the forms "Brett" and "Britt" were used. The spellings "Bret" and "Brit" were used in Scots.
The first Professor of Celtic at the University of Oxford, John Rhys, was the first person to use the name "Brython" (from ) in academia as another English language word for the Britons. He named the languages of the Britons "Brythonic", or the "Brythonic languages". Rhys used these names to be clear about what he meant, because names like "British" can be ambiguous ("with more than one meaning"). "Brythonic" and "Brythonic languages" are names that are no longer common. The names "Brittonic" and "Brittonic languages" are now more common in linguistics ("the study of languages").
Celtic tribes.
From the Iron Age onward, Britain was divided among a variety of Celtic tribes. When the Romans came, many of the tribes adopted Roman culture and the Latin language.