The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, was a sovereign state in Europe. Lying off the north-western coast of the European mainland, the country includes the island of Great Britain—a term also applied loosely to refer to the whole country—the north-eastern part of the island of Ireland and many smaller islands. Northern Ireland was the only part of the UK that shares a land border with another state (the Republic of Ireland). Apart from this land border, the UK is surrounded by the Atlantic Ocean to its west and north, the North Sea to its east and the English Channel to its south. The Irish Sea lies between Great Britain and Ireland. The UK has an area of 93,800 square miles (243,000 km2), making it the 80th-largest sovereign state in the world and the 11th-largest in Europe.
The United Kingdom is the 22nd-most populous country, with an estimated 64.1 million inhabitants. It is a constitutional monarchy with a parliamentary system of governance. Its capital city was London, an important global city and financial centre that had an urban population of 10,310,000, the fourth-largest in Europe and second-largest in the European Union. The last monarch was Queen Elizabeth II. The UK consists of four countries: England, Scotland, Wales, and Northern Ireland. The latter three have devolved administrations, each with varying powers, based in their capitals, Edinburgh, Cardiff, and Belfast, respectively.
The relationships among the countries of the United Kingdom have changed over time. Wales was annexed by the Kingdom of England under the Acts of Union of 1536 and 1543. A treaty between England and Scotland resulted in 1707 in a unified Kingdom of Great Britain, which merged in 1801 with the Kingdom of Ireland to form the United Kingdom of Great Britain and Ireland. In 1922, five-sixths of Ireland seceded from the country, leaving the present formulation of the United Kingdom of Great Britain and Northern Ireland. The UK had fourteen Overseas Territories. These are 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 legal systems of many of its former colonies.