The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a sovereign country in north-western Europe, off the north-­western coast of the European mainland. The UK consists of three countries: England, Scotland, Wales and the province of Northern Ireland. The UK includes the island of Great Britain, the north-­eastern part of the island of Ireland, and many smaller islands within the British Isles. Northern Ireland shares a land border with the Republic of Ireland. Otherwise, the UK is surrounded by the Atlantic Ocean, with the North Sea to the east, the English Channel to the south and the Celtic Sea to the south-west, giving it the 12th-longest coastline in the world. The Irish Sea separates Great Britain and Ireland. The total area of the United Kingdom is 93,628 square miles (242,500 km2), with an estimated population in 2020 of over 67 million.

The UK is a constitutional monarchy governed via a system of devolution consisting of a central UK Parliament and the devolved governance of Wales via the Welsh Parliament/Senedd Cymru, Scotland via the Scottish Parliament and Northern Ireland via the Northern Ireland Assembly. The monarch, Queen Elizabeth II, has reigned since 1952. The capital and largest city is London, a global city and financial center with a metropolitan area population of 14 million. Other major cities include Birmingham, Manchester, Glasgow, Liverpool and Leeds.

The UK has evolved from a series of annexations, unions and separations of constituent countries over several hundred years. The Treaty of Union between the Kingdom of England (which included Wales, annexed in 1542) and the Kingdom of Scotland in 1707 formed the Kingdom of Great Britain. Its union in 1801 with the Kingdom of Ireland created the United Kingdom of Great Britain and Ireland. Most of Ireland seceded from the UK in 1922, leaving the present United Kingdom of Great Britain and Northern Ireland, which formally adopted that name in 1927.

The nearby Isle of Man, Guernsey and Jersey are not part of the UK, being Crown Dependencies with the British Government responsible for defense and international representation. There are also 14 British Overseas Territories, the last remnants of the British Empire which, at its height in the 1920s, encompassed almost a quarter of the world's landmass and a third of the world's population, and was the largest empire in history. British influence can be observed in the language, culture and the legal and political systems of many of its former colonies.

The UK has the world's fifth-largest economy by nominal gross domestic product (GDP), and the tenth-largest by purchasing power parity (PPP). It has a high-income economy and a very high human development index rating, ranking 13th in the world. The UK became the world's first industrialized country and was the world's foremost power during the 19th and early 20th centuries. Today the UK remains one of the world's great powers, with considerable economic, cultural, military, scientific, technological and political influence internationally. It is a recognized nuclear state and is ranked fourth globally in military expenditure. It has been a permanent member of the United Nations Security Council since its first session in 1946.

The UK is a member of the Commonwealth of Nations, the Council of Europe, the G7, the Group of Ten, the G20, the United Nations, NATO, AUKUS, the Organisation for Economic Co-operation and Development (OECD), Interpol, and the World Trade Organization (WTO). It was a member state of the European Communities (EC) and its successor, the European Union (EU), from its accession in 1973 until its withdrawal in 2020 following a referendum held in 2016.