The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom, the UK, Great Britain, or Britain, is an island country located off the northwestern coast of mainland Europe comprising of the four constituent countries; England, Scotland, Wales and Northern Ireland. It comprises the island of Great Britain, the northeast part of the island of Ireland and many small islands. Northern Ireland is the only part of the UK with a land border, sharing it with the Republic of Ireland. Apart from this land border, the UK is surrounded by the Atlantic Ocean, the North Sea, the English Channel and the Irish Sea. The largest island, Great Britain, is linked to France by the Channel Tunnel.

The United Kingdom is a parliamentary democracy and a constitutional monarchy with Queen Elizabeth II as head of state. The Crown Dependencies of the Channel Islands and the Isle of Man, formally possessions of the Crown, are not part of the UK but form a federacy with it. The UK has fourteen overseas territories, all remnants of the British Empire, which at its height encompassed almost a quarter of the world's land surface, making it the largest empire in history. As a direct result of the empire, British influence can be observed in the infrastructure, culture, sporting preferences and language of many other leading countries all over the globe including Australia, Canada, New Zealand and the United States of America as well as in less globally influential independent states. Queen Elizabeth II remains the head of the Commonwealth of Nations and head of state of the Commonwealth realms. It is a developed country, with the fifth or sixth largest economy depending on source.

The UK was the world's foremost power during the 19th and early 20th century, but the economic cost of two world wars and the decline of its empire in the latter half of the 20th century diminished its leading role in global affairs. The UK nevertheless retains significant economic, cultural, military and political influence and is a nuclear power, with the second highest defence spending in the world. It holds a permanent seat on the United Nations Security Council, and is a member of the G8, NATO, the European Union and the Commonwealth of Nations.