The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is an island country in Northwestern Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales, and Northern Ireland. It includes the island of Great Britain, the north-eastern part of the island of Ireland, and most of the 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 93,628 square miles (242,495 km2), with an estimated 2022 population of nearly 67 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.
The UK became the first industrialised country and was the world's foremost power for the majority of the 19th and early 20th centuries, particularly during the "Pax Britannica" between 1815 and 1914. The British Empire, at its height in the 1920s, encompassed almost a quarter of the world's landmass and population, and was the largest empire in history; however, its involvement in the First World War and the Second World War damaged Britain's economic power and a global wave of decolonisation led to the independence of most British colonies. British influence can be observed in the legal and political systems of many of its former colonies, and the UK's culture remains globally influential, particularly in literature, music and sport.
The United Kingdom is a constitutional monarchy and parliamentary democracy. The capital and largest city of the United Kingdom (as well as the capital of England) is London. The cities of Edinburgh, Cardiff, and Belfast are respectively the national capitals of Scotland, Wales, and Northern Ireland. Other major cities include Birmingham, Manchester, Glasgow, and Leeds. The UK consists of three distinct legal jurisdictions: England and Wales, Scotland, and Northern Ireland. This is due to these areas retaining their existing legal systems even after joining the UK. Since 1998, Scotland, Wales, and Northern Ireland also have their own devolved governments and legislatures, each with varying powers.
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. The UK has been a permanent member of the UN Security Council since its first session in 1946. It is a member of the Commonwealth of Nations, the Council of Europe, the G7, the OECD, NATO, the Five Eyes, AUKUS and the CPTPP.