A Briton is a member of Celtic tribes which inhabited Britain before, during, and after the Roman occupation. The Welsh and Cornish are the only modern representatives of Britons in Great Britain, although the Bretons of Brittany or Bretagne descend from British refugees who fled Britain after the end of Roman rule.
Britons spoke Brythonic or Brittonic, a P-Celtic language. Welsh and Breton are modern representatives. Attempts are being made to revive the Cornish language.
Scots Gaelic and Manx Gaelic are Q-Celtic or Goidelic. It remains unknown whether the Picts were Celts or what language they spoke.
A Briton is also a citizen of the United Kingdom - see British, Alternate words for British.