The United Kingdom (UK ), officially the United Kingdom of Great Britain and Northern Ireland, informally as Britain, is a sovereign country lying off the north-western coast of the European mainland. The United Kingdom includes the island of Great Britain, the north-eastern part of the island of Ireland and many smaller islands. Northern Ireland is the only part of the United Kingdom that shares a land border with another sovereign state, the Republic of Ireland. Apart from this land border, the United Kingdom 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 lies between Great Britain and Ireland. With an area of 242500 km2, the United Kingdom is the 78th-largest sovereign state in the world. It is also the 22nd-most populous country, with an estimated 66.0 million inhabitants in 2017.
The UK is a unitary parliamentary democracy and constitutional monarchy. The current monarch is Queen Elizabeth II, who has reigned since 1952, making her the longest-serving current head of state. The United Kingdom's capital and largest city is London, a global city and financial centre with an urban area population of 10.3 million. Other major urban areas in the UK include Manchester, Birmingham, Leeds, Glasgow and Liverpool.
The United Kingdom consists of four constituent countries: England, Scotland, Wales, and Northern Ireland. Their capitals are London, Edinburgh, Cardiff, and Belfast respectively. Apart from England, the countries have their own devolved governments, each with varying powers. The nearby Isle of Man, Bailiwick of Guernsey and Bailiwick of Jersey are not part of the UK, being Crown dependencies with the British Government responsible for defence and international representation. The medieval conquest and subsequent annexation of Wales by the Kingdom of England, followed by the union between England and Scotland in 1707 to form the Kingdom of Great Britain, and the union in 1801 of Great Britain with the Kingdom of Ireland created the United Kingdom of Great Britain and Ireland. Five-sixths of Ireland seceded from the UK in 1922, leaving the present formulation of the United Kingdom of Great Britain and Northern Ireland. There are fourteen British Overseas Territories, the remnants of the British Empire which, at its height in the 1920s, encompassed almost a quarter of the world's land mass and was the largest empire in history. British influence can be observed in the language, culture and political systems of many of its former colonies.
The United Kingdom is a developed country and has the world's fifth-largest economy by nominal GDP and ninth-largest economy by purchasing power parity. It has a high-income economy and has a very high Human Development Index rating, ranking 14th in the world. It was the world's first industrialised country and the world's foremost power during the 19th and early 20th centuries. The UK remains a great power, with considerable economic, cultural, military, scientific and political influence internationally. It is a recognised nuclear weapons state and is sixth in military expenditure in the world. It has been a permanent member of the United Nations Security Council since its first session in 1946. It has been a leading member state of the European Union (EU) and its predecessor, the European Economic Community (EEC), since 1973; however, a referendum in 2016 resulted in 51.9 per cent of UK voters favouring leaving the European Union, and the country's exit is being negotiated. The United Kingdom is also a member of the Commonwealth of Nations, the Council of Europe, the G7, the G20, NATO, the Organisation for Economic Co-operation and Development (OECD), Interpol and the World Trade Organization (WTO).
The 1707 Acts of Union declared that the kingdoms of England and Scotland were "United into One Kingdom by the Name of Great Britain". The term "United Kingdom" has occasionally been used as a description for the former kingdom of Great Britain, although its official name from 1707 to 1800 was simply "Great Britain". The Acts of Union 1800 united the kingdom of Great Britain and the kingdom of Ireland in 1801, forming the United Kingdom of Great Britain and Ireland. Following the partition of Ireland and the independence of the Irish Free State in 1922, which left Northern Ireland as the only part of the island of Ireland within the United Kingdom, the name was changed to the "United Kingdom of Great Britain and Northern Ireland".
Although the United Kingdom is a sovereign country, England, Scotland, Wales and Northern Ireland are also widely referred to as countries. The UK Prime Minister's website has used the phrase "countries within a country" to describe the United Kingdom. Some statistical summaries, such as those for the twelve NUTS 1 regions of the United Kingdom refer to Scotland, Wales and Northern Ireland as "regions". Northern Ireland is also referred to as a "province". With regard to Northern Ireland, the descriptive name used "can be controversial, with the choice often revealing one's political preferences".
The term "Great Britain" conventionally refers to the island of Great Britain, or politically to England, Scotland and Wales in combination. However, it is sometimes used as a loose synonym for the United Kingdom as a whole.
The term "Britain" is used both as a synonym for Great Britain, and as a synonym for the United Kingdom. Usage is mixed, with the BBC preferring to use Britain as shorthand only for Great Britain and the UK Government, while accepting that both terms refer to the United Kingdom, preferring to use the term UK rather than Britain. The UK Permanent Committee on Geographical Names lists "United Kingdom" and "UK or U.K." as shortened and abbreviated geopolitical terms for the United Kingdom of Great Britain and Northern Ireland, it does not list "Britain", stating that it has been used "informally" by government websites.
The adjective "British" is commonly used to refer to matters relating to the United Kingdom. The term has no definite legal connotation, but is used in law to refer to United Kingdom citizenship and matters to do with nationality. People of the United Kingdom use a number of different terms to describe their national identity and may identify themselves as being British, English, Scottish, Welsh, Northern Irish, or Irish; or as belonging to a combination of different national identities.