Evolution of the words
The meanings of Britain and British have evolved over time and as they have gained political significance.
In 325 BC the Greek Pytheas of Massalia visited a group of islands which he called Pretaniké, the principal ones being Albionon (Albion) and Ierne (Erin). (The records of this visit date from much more recent times, so there is room for these details to be disputed.) To linguists, this suggests the Brythonic inhabitants called themselves Priteni.
In manuscripts of the Anglo-Saxon chronicle there is a reference to the inhabitants having migrated to the islands from "Armenia" but most historians believe this was a mistake in transcription and that the actual origin of the islanders was Armorica.
Because of resistance to Roman rule in Armorica (which was supported by Celtic aristocrats in the islands) Julius Caesar responded with two invasions of the main island in 55 and 54 BC.
Some believe that when the Romans took over the southern part of Great Britain they named the island after the Brigantes, one of the largest Celtic Tribes.
However the Romans derived the name, they called their possessions Britannia. The earlier Celtic inhabitants became known as Britons and the island as Britain. After the fall of the Roman Empire, the name Britannia largely fell into abeyance and tended to be used in an historical sense, referring to the Roman possessions.
Some centuries after the Romans had left, some of the Britons returned to the near continent. Further centuries later Geoffrey of Monmouth used the names Britannia minor to refer to the Armorican region they had returned to and Britannia major for the island.
Only by the late Middle Ages did the concept of "Britain" come to represent anything more modern than the Romans. By then, the continental region was known as Brittany and the island as Great Britain (compare the French names Bretagne and Grande Bretagne).
The kingdoms established on the island of Great Britain were perceived to be dominant over the whole archipelago, which was known as the British Isles. During the reign of Queen Elizabeth I the queen's astrologer and alchemist, John Dee, wrote mystical volumes predicting a British Empire and using the terms "Great Britain" and "Britannia". After Elizabeth's death in 1603 the kingdoms shared one King, James VI of Scotland and I of England. On 20 October 1604 he proclaimed himself "King of Great Britain" (thus including Wales and also avoiding the cumbersome title "King of England and Scotland"). This title was eventually adopted formally in 1707 when the Kingdom of Great Britain was formed. The adjective used for the kingdom was British.
Since its formation, the kingdom was enlarged in 1801 by the addition of the island of Ireland, then reduced in 1920 by the independence of what is now the Republic of Ireland. The name of the kingdom changed accordingly, becoming The United Kingdom of Great Britain and Northern Ireland. To some writers the meaning of British and Britain have changed with the Kingdom. The word British is now in common use to indicate United Kingdom (UK) nationality because there is no suitable substitute. However, to other writers Britain is still synonymous with only the island of Great Britain.
Other terms also cause confusion. Great Britain is undisputedly the name of the large island, but is occasionally used to mean the UK, for instance in the modern Olympic Games. The British Isles is still a geographical term for the archipelago, but it can also still be seen as implying dominance by Great Britain, so it is sometimes avoided. The prefix Anglo, historically meaning English, is sometimes used to denote the UK, as in Anglo-Irish. See the respective articles.