The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom, the UK or Britain,^[11][12] is a sovereign state located off the northwestern coast of continental Europe. It is an island country,^[13][14] spanning the island of Great Britain, the northeast part of the island of Ireland, and many small islands. Northern Ireland is the only part of the UK with a land border, sharing it with the Republic of Ireland.^[15] Apart from this land border, the UK is surrounded by the Atlantic Ocean, the North Sea, the English Channel and the Irish Sea. The largest island, Great Britain, is linked to France by the Channel Tunnel.

The United Kingdom is a unitary state consisting of four countries: England, Northern Ireland, Scotland and Wales.^[16] It is governed by a parliamentary system with its seat of government in London, the capital, but with three devolved national administrations in Belfast, Cardiff and Edinburgh, the capitals of Northern Ireland, Wales and Scotland respectively. The UK is a constitutional monarchy with Queen Elizabeth II as the head of state. The Channel Islands and the Isle of Man are Crown Dependencies and not part of the UK, but form a federacy with it.^[17] The UK has fourteen overseas territories,^[18] all remnants of the British Empire, which at its height encompassed almost a quarter of the world's land surface, the largest empire in history. British influence can continue to be observed in the language, culture and legal systems of many of its former colonies. Queen Elizabeth II remains the head of the Commonwealth of Nations and head of state of each of the Commonwealth realms.

The UK is a developed country, with the fifth (nominal GDP) or sixth (PPP) largest economy in the world. It was the world's first industrialised country^[19] and the world's foremost power during the 19th and early 20th centuries,^[20] but the economic cost of two world wars and the decline of its empire in the latter half of the 20th century diminished its leading role in global affairs. The UK nevertheless remains a major power with strong economic, cultural, military and political influence and is a nuclear power, with the second highest defence spending in the world. It is a Member State of the European Union, holds a permanent seat on the United Nations Security Council, and is a member of the G8, NATO, Organisation for Economic Co-operation and Development, World Trade Organization and the Commonwealth of Nations.