The United Kingdom of Great Britain and Northern Ireland (commonly known as the United Kingdom, the U.K., or Britain) is a country to the northwest of mainland Europe. 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 comprising four constituent countries — England, Northern Ireland, Scotland and Wales — with 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. However, 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 largest economy in the world by nominal GDP.
Britain 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.
The United Kingdom of Great Britain and Northern Ireland is in Western Europe. It comprises the island of Great Britain (most of England, Scotland and Wales) and the northeastern one-sixth of the island of Ireland (Northern Ireland), together with many smaller islands. The Royal Greenwich Observatory, near London, is the defining point of the Prime Meridian. The United Kingdom has a total area of approximately 245,000 square kilometers. The UK lies between the North Atlantic Ocean and the North Sea, and comes within 35 kilometres of the northwest coast of France, from which it is separated by the English Channel. Northern Ireland shares a 360 kilometres land boundary with the Republic of Ireland. The Channel Tunnel ("Chunnel") now links the UK with France beneath the English Channel.
All parts of the United Kingdom have a temperate climate, with plentiful rainfall all year round. The temperature varies with the seasons but seldom drops below ?10 C or rises above 35 C. The prevailing wind is from the southwest, bearing frequent spells of mild and wet weather from the Atlantic Ocean. Eastern parts are most sheltered from this wind and are therefore the driest. Atlantic currents, warmed by the Gulf Stream, bring mild winters, especially in the west, where winters are also wet, especially over high ground. Summers are warmest in the south east of England, being closest to the European mainland, and coolest in the north. Snowfall can occur in winter and early spring, though it rarely settles to any great depth away from high ground.