The United Kingdom of Great Britain and Northern Ireland (usually shortened to the United Kingdom, the U.K., or Britain) is a country and sovereign state that lies to the northwest of mainland Europe, with its only land border with the Republic of Ireland. It extends over all of the island of Great Britain and the north-east part of the island of Ireland.

The United Kingdom is bounded by the Atlantic Ocean, and its ancillary bodies of water, including the North Sea, the English Channel, the Celtic Sea, St George's Channel, and the Irish Sea. The United Kingdom is linked to France and Continental Europe by the Channel Tunnel.

The United Kingdom is a constitutional monarchy composed of four constituent countries: England, Scotland, Wales and Northern Ireland. The current monarch is Queen Elizabeth II, who is also the Queen and Head of State of fifteen other Commonwealth Realms, including Canada, Australia, New Zealand and Jamaica. The Crown Dependencies of the Channel Islands and the Isle of Man, formally possessions of the Crown, form a federacy with the United Kingdom collectively known as the British Islands. The UK also has fourteen overseas territories, all remnants of the British Empire which at its height encompassed more than a quarter of the world's surface and population.

Although Britain was the foremost great power during the 19th century, the economic cost of two world wars and the decline of it's empire during the latter half of the 20th century diminished Britain's status in global affairs. However, as a permanent member of the United Nations Security Council, a nuclear power, a member of the G8, the world's fifth largest economy, and having the second highest defence spending, Britain remains an important political, economic and military world power. It is a member of the European Union and the Commonwealth of Nations.

The United Kingdom contains some of the world's leading seats of higher education, such as the universities of Oxford and Cambridge, along with Imperial College, the London School of Economics, King's College London (KCL) and University College (UCL) of the University of London.

In 2007 there are 112 universities/university colleges in the UK. This amounts to 138 university-level institutions if the distinct constituent colleges of the University of London and University of Wales are defined as separate universities. Most of the UK's major cities are home to two or more universities.

Parts of the United Kingdom use a segregation system in their state-sector schooling. In these areas parents may choose to enter their children into the Eleven Plus, an entrance test to a specific group of elite state Grammar schools. However some Grammar Schools have an additional test named an Entrance Exam, as well as the Eleven Plus. But those who do not take the test, or do not reach the mark necessary for entrance to these schools, are usually allocated a place at a comprehensive school, where children are not selected on the basis of academic aptitude. This is a controversial system as it gives greater opportunities to high-achieving students possibly at the expense of other students. This has lead some to accuse the grammar school system of promoting elitism.

The United Kingdom also has an extensive private education system, with some of these independent schools confusingly known as Public Schools.