The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and Northern Ireland. The United Kingdom 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 United Kingdom is surrounded by the Atlantic Ocean, the North Sea, the English Channel, the Celtic Sea and the Irish Sea. The total area of the United Kingdom is 242,495 square kilometres (93,628 sq mi), with an estimated 2023 population of over 68 million people.

The United Kingdom 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 also included Wales) and the Kingdom of Scotland in 1707 resulted in their unification to become 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 its name in 1927. The nearby Isle of Man, Guernsey and Jersey are not part of the UK, being Crown Dependencies, but the British government is responsible for their defence 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 population, and was the largest empire in history. A part of the core Anglophonic world, British influence can be observed in the language, culture, legal and political systems of many of its former colonies.

The United Kingdom is a constitutional monarchy and parliamentary democracy. Its capital and largest city is London, the capital of England, a global city and financial centre with a population of over 14 million people. Edinburgh, Cardiff and Belfast are the national capitals of Scotland, Wales and Northern Ireland respectively. Other major cities include Birmingham, Manchester, Leeds, Glasgow and Liverpool. Scotland, Wales, and Northern Ireland have their own devolved governments, each with varying powers. The UK became the world's first industrialised country and was the foremost power during the 19th and early 20th centuries, a period of unchallenged global hegemony known as "Pax Britannica". In the 21st century, the UK remains a great power and has significant economic, cultural, military, scientific, technological and political influence. The UK has the world's sixth-largest economy by nominal gross domestic product (GDP), and the tenth-largest by purchasing power parity. It is a recognised nuclear state and is ranked fourth globally in military expenditure, and as of 2022 has the highest total military spending of any country in Europe. It has been a permanent member of the United Nations Security Council since its first session in 1946.

The United Kingdom is a member of the Commonwealth of Nations, the Council of Europe, the G7, the Group of Ten, the G20, Five Eyes, the United Nations (UN), NATO, AUKUS, the Organisation for Economic Co-operation and Development (OECD), Interpol, and the World Trade Organization (WTO). The UK is set to join the CPTPP, a major trade bloc in the Indo-Pacific, after negotiations regarding its ascension concluded in March 2023. The UK is also considered a part of the "Big Four", or G4, an unofficial group of four European nations. 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.