Where did the world "Britain" come from?
Well, there area lot of theories as to where that word came from. No-one knws for sure.
---------------
The word Britain is an informal term used to refer to the island of Great Britain which consists of the nations of England, Scotland and Wales.
The United Kingdom of Great Britain and Northern Ireland or UK,
sometimes the Roman province called "Britain" or "Britannia"
The word British generally means belonging to or associated with Britain in one of the first two senses above (i.e. the United Kingdom or the island of Great Britain).
However, the term has a range of related usages, as described in this article.
Etymologically, these words are closely related to Brittany, the name of the western French peninsula, and its adjective Breton.
Earliest attested references
Pretaniké; Pretanikai nesoi (Pretanic isles) - 325 BC
Britannia - 55 BC (Julius Caesar, Roman invasion of Britain)
Breten - 855 (Old English Chronicle, introduction)
Brittisc - 855 (OED)
Grate Briteigne - 1548 (OED)
British isles - 1550 (in Latin; map of Sebastian Munster cited in British Isles article)
Etymology
The etymology of the name Britain is thought to derive from a Celtic word, Pritani, "painted people/men", a reference to the inhabitants of the islands' use of body-paint and tattoos. If this is true, there is an interesting parallel with the name Pict, connected with a Latin word of the same meaning. The modern Welsh name for Britain is Prydain. The Q-Celtic form was Cruithin, showing that the Common Celtic singular form was qr[ui]tanos. The root is presumably that of the modern Gaelic/Irish word cruth 'shape, form'.
It has also been postulated that Britain may derive from the Celtic goddess Brigid; the form of the word, however, is against this postulation.
In 325 BC the Greek explorer 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, but it does seem to attest pre-Roman use of the name by Celtic-speaking inhabitants of the islands - or the names used by the Phoenecians Pytheas went with.
The Roman geographer Ptolemy called the larger island Megale Brettania (Great Britain), and the smaller island Micra Bretannia (Little Britain).
Britain and Brittany
The original reference seems to have been to the territory in which the Brythonic languages were spoken, which more or less coincided with the Roman province of Britannia, an area equivalent to modern England, Wales and southern Scotland. In the Early Middle Ages speakers of a Brythonic language which later evolved into Breton migrated from Cornwall to Armorica, Western France, possibly because of pressure from Saxon invasions. This is why different forms of the same name apply to insular Britain and continental Brittany. In French the similarity is even more obvious: Bretagne and Grande Bretagne.
Geoffrey of Monmouth used the names Britannia minor to refer to the Armorican region and Britannia major for the island. The element great in the term Great Britain thus simply means large, to make the distinction from Brittany.
Historical evolution of the term Britain
The kingdoms established on the island of Great Britain were perceived to be dominant over the whole archipelago, which thus came to be known as the British Isles. During the reign of Queen Elizabeth I of England, 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 Brittaine" (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.
Politically, then, British has been used to described someone or something from the United Kingdom, in its various forms, since 1707. Briton or Brit are also used colloquially in this form, though the use of Briton here is incorrect.
Since its formation, the kingdom was enlarged in 1801 by the addition of the island of Ireland - already ruled by the British monarchy - to become the United Kingdom of Great Britain and Ireland, and was then reduced in 1922 by the independence of the Irish Free State, now the Republic of Ireland. The name of the kingdom changed accordingly, in 1927 becoming The United Kingdom of Great Britain and Northern Ireland.
British was also used to describe members of nations that formed part of the British Empire. This use now, however, could be seen as justifying the colonial era, even if only applied historically.
Modern use of the term 'British'
The modern use of the term 'British' is as an adjective to describe someone or something from the United Kingdom. It is officially used as the term to describe the nationality of a citizen of the United Kingdom. Irish Nationalists may reject this term as offensive, as it is used to describe Irish people in Northern Ireland. Many people from England, Scotland and Wales also dislike the term, preferring to define themselves as natives of their own particular country.
It is also frequently used to describe residents of the United Kingdom's current colonies. This may still offend some people, though since the British Overseas Territories Act 2002 all residents of the United Kingdom's remaining colonies have been eligible for British citizenship, making the term more apt.
British occurs in the legal term British Islands . This was coined to describe all of the islands of the British Isles, exlcuding those that form part of the Republic of Ireland, when they act together as a political whole.
Geographically, the term can be used in various ways:
To describe someone from the island of Great Britain
In the term British Isles, the traditional term for the entire archipelago of islands that lie off the north west coast of France, of which Great Britain and Ireland are the two biggest. Note that this is not intended to imply that all of these islands are part of the United Kingdom, for many of them are part of the Republic of Ireland. However, confusion caused by this term can lead to offense.
The term has historically been used to describe someone or something from the British Isles. Due to the above mentioned potential for offense, this rarely happens today. For example the British Lions a rugby team which draws players from the United Kingdom and the Republic of Ireland has been renamed to the British and Irish Lions.
Sometimes British applies to an area or territory currently or formerly governed by or a dependent territory of the United Kingdom, for example the British Virgin Islands, the British Indian Ocean Territory, or British Columbia which is now a province of Canada.
Brutus of Troy
In keeping with the mediaeval penchant for etymologising country names in terms of eponomous heroes, English historians of the late mediaeval and early modern periods charted the history of the nation from Brutus of Troy, supposedly a hero of the Trojan war who founded Britain just as Aeneaus' descendant Romulus founded Rome, Frankus France, and so forth. The life of Brutus, anglicised as Brute, was recorded in the literary tradition of the Prose Brute. This was long accepted as the etymology of Britain.
http://en.wikipedia.org/wiki/Britain