History of the term
The earliest known reference to the inhabitants of Great Britain may have come from 4th century BC records of the voyage of Pytheas, a Greek geographer who made a voyage of exploration around the British Isles. Although none of his own writings remain, writers during the time of the Roman Empire made much reference to them. Pytheas called the islands collectively αἱ Βρεττανίαι (hai Brettaniai), which has been translated as the Brittanic Isles, and the peoples of what are today England, Wales, Scotland and the Isle of Man of Prettanike were called the Πρεττανοί (Prettanoi), Priteni, Pritani or Pretani.
The group included Ireland, which was referred to as Ierne (Insula sacra "sacred island" as the Greeks interpreted it) "inhabited by the different race of Hiberni" (gens hibernorum), and Britain as insula Albionum, "island of the Albions". The term Pritani may have reached Pytheas from the Gauls, who possibly used it as their term for the inhabitants of the islands.
Greek and Roman writers, in the 1st century BC and the 1st century AD, name the inhabitants of Great Britain and Ireland as the Priteni, the origin of the Latin word Britanni. It has been suggested that this name derives from a Gaulish description translated as "people of the forms", referring to the custom of tattooing or painting their bodies with blue woad made from Isatis tinctoria. Parthenius, a 1st-century Ancient Greek grammarian, and the Etymologicum Genuinum, a 9th-century lexical encyclopaedia, mention a mythical character Bretannus (the Latinised form of the Ancient Greek: Βρεττανός, Brettanós) as the father of Celtine, mother of Celtus, the eponymous ancestor of the Celts.
By 50 BC Greek geographers were using equivalents of Prettanikē as a collective name for the British Isles. However, with the Roman conquest of Britain the Latin term Britannia was used for the island of Great Britain, and later Roman-occupied Britain south of Caledonia, although the people of Caledonia and the north were also the self same Britons during the Roman period, the Gaels arriving four centuries later. Following the end of Roman rule in Britain, the island of Great Britain was left open to invasion by pagan, seafaring warriors such as Germanic-speaking Anglo-Saxons and Jutes from Continental Europe, who gained control in areas around the south east, and to Middle Irish-speaking people migrating from what is today Northern Ireland to the north of Great Britain (modern Scotland), founding Gaelic kingdoms such as Dál Riata and Alba, which would eventually subsume the native Brittonic and Pictish kingdoms and become Scotland.
In this sub-Roman Britain, as Anglo-Saxon culture spread across southern and eastern Britain and Gaelic through much of the north, the demonym "Briton" became restricted to the Brittonic-speaking inhabitants of what would later be called Wales, Cornwall, North West England (Cumbria), and parts of Scotland such as Strathearn, Morayshire, Aberdeenshire and Strathclyde. In addition the term was also applied to Brittany in what is today France and Britonia in north west Spain, both regions having been colonised by Britons in the 5th century fleeing the Anglo-Saxon invasions. However, the term Britannia persisted as the Latin name for the island. The Historia Brittonum claimed legendary origins as a prestigious genealogy for Brittonic kings, followed by the Historia Regum Britanniae which popularised this pseudo-history to support the claims of the Kings of England.
During the Middle Ages, and particularly in the Tudor period, the term "British" was used to refer to the Welsh people and Cornish people. At that time, it was "the long held belief that these were the remaining descendants of the Britons and that they spoke 'the British tongue'". This notion was supported by texts such as the Historia Regum Britanniae, a pseudohistorical account of ancient British history, written in the mid-12th century by Geoffrey of Monmouth. The Historia Regum Britanniae chronicled the lives of legendary kings of the Britons in a narrative spanning 2000 years, beginning with the Trojans founding the ancient British nation and continuing until the Anglo-Saxon settlement of Britain in the 7th century forced the Britons to the west, i.e. Wales and Cornwall, and north, i.e. Cumbria, Strathclyde and northern Scotland. This legendary Celtic history of Great Britain is known as the Matter of Britain. The Matter of Britain, a national myth, was retold or reinterpreted in works by Gerald of Wales, a Cambro-Norman chronicler who in the 12th and 13th centuries used the term British to refer to the people later known as the Welsh.