The United Kingdom of Great Britain and Northern Ireland (usually shortened to the United Kingdom, the U.K., or Britain is a country and sovereign state to the northwest of mainland Europe. It comprises the island of Great Britain, the north-east part of the island of Ireland and many small local islands. Northern Ireland is the only part of the UK with a land border, sharing it with the Republic of Ireland Apart from this land border, the UK is surrounded by the waters of the Atlantic Ocean, the North Sea, the English Channel, and the Irish Sea. It is linked to France by the Channel Tunnel.

The United Kingdom is a constitutional monarchy comprising four constituent countries: England, Northern Ireland, Scotland and Wales. The current monarch is Queen Elizabeth II who is also Queen and Head of State of fifteen other Commonwealth Realms including Canada, Australia, New Zealand and Jamaica. The Crown Dependencies of the Channel Islands and the Isle of Man, formally possessions of the Crown, are not part of the UK but form a federacy with it. The UK has fourteen overseas territories, all remnants of the British Empire, which at its height encompassed almost a quarter of the world's land surface.