The United Kingdom of Great Britain and Northern Ireland is a country in western Europe. It is a Commonwealth Realm, and a member of the European Union and NATO. Usually known simply as the United Kingdom or the UK, it is also often inaccurately named Great Britain, Britain or England (the most populous of the home nations). The UK has four constituent parts, three of which the ancient nations of England, Wales and Scotland are located on the island of Great Britain. The fourth part is Northern Ireland, which is located on the island of Ireland.
The border between Northern Ireland and the Republic of Ireland forms the United Kingdom's principal international land border, although there is also a nominal frontier with France in the middle of the Channel Tunnel. The UK also has overseas territories throughout the world, and relationships with several Crown Dependencies.
The UK was formed by a series of Acts of Union which united the Kingdom of England (which included Wales as a principality) with those of, first, Kingdom of Scotland and then Kingdom of Ireland under a single government in London. The greater part of Ireland left the United Kingdom (then called the United Kingdom of Great Britain and Ireland) in 1922 to form an independent state (in which, until 1949, the King of the United Kingdom was also King of Ireland). This state later became the Republic of Ireland. Six counties in the north-eastern portion of the island, meanwhile, remained a part of the United Kingdom, forming Northern Ireland to this day.
The UK is situated off the north-western coast of continental Europe, and has a land border with the Republic of Ireland, but is otherwise surrounded by the North Sea, the English Channel, the Celtic Sea, the Irish Sea, and the Atlantic Ocean.
Great Britain, or just Britain, is the geographical name of the largest of the British Isles (often also including its smaller neighbouring islands, though never Ireland). Politically, the term Great Britain refers collectively to the nations of England, Wales and Scotland (i.e., the United Kingdom except for Northern Ireland). This political usage of "Great Britain" dates from the personal union of the Crowns of Scotland and England (including Wales) in 1603, with the term being used in the sense "all of Britain". In the early years of the "United Kingdom of Great Britain", formed by the Act of Union of 1707, it was customary to refer officially to Scotland and to England and Wales as, respectively, "North Britain" and "South Britain", though the usage never really caught on. It should be noted that the practice by some, the informal media in particular, of using "(Great) Britain" as shorthand for the United Kingdom is an inaccuracy which can cause offence.
The British Isles is a term frequently used to refer to the archipelago which includes the mainland of Great Britain, the mainland of Ireland, and the smaller islands associated with these two, such as the Channel Islands, the Hebrides, the Isle of Man, the Isle of Wight, the Orkney Isles, the Shetland Islands, etc. The term is, however, often avoided, especially in Ireland, by those who are conscious that it is sometimes misunderstood internationally to mean "the islands belonging to Britain (i.e. the United Kingdom)", a description out of date in the Irish case since 1922. An alternative, the Islands of the North Atlantic (IONA) has been proposed, but is little used outside diplomatic circles. But hey!! Who gives a damn?