The Ancient Britons were those ancient inhabitants of the island of Great Britain who spoke the Celtic Common Brittonic language, which diversified into a group of related Celtic languages such as Welsh, Cornish, Pictish, Cumbric and Breton. The Britons lived all over the island of Great Britain and on the surrounding islands and archipelagos such as the Orkneys, Shetlands, Hebrides and Isle of Man. Ireland was inhabited by a different group of Celts, speaking Goidelic (or Gaelic).
The coming of the Anglo-Saxons and Gaelic speaking Celts from the 5th century AD onwards, and the resulting gradual spread of the collection of dialects that would become the English language and Scots Gaelic, between them eventually extinguished Brittonic from much of its former territory by the 12th century AD, leaving Brittonic speakers only in Wales, Cornwall and Brittany.
The term is usually used in reference to the people of Great Britain in the Iron Age (from approximately the 7th century BC), and through the Roman, Sub-Roman period, Middle Ages and the Tudor period, although there is thought to have been little or no change to the genetic mixture at the start of the period, and relatively little by the end. During the 18th century however, and particularly after the Acts of Union 1707 the terms British and Briton would gradually come to be applied not just to the remaining Brittonic peoples themselves, but to all of the inhabitants of the United Kingdom, including the English, Scottish and Northern Irish.