The United Kingdom of Great Britain and Northern Ireland (commonly known as the United Kingdom, the UK, or Britain) is a sovereign state located off the north-western coast of continental Europe. The country 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 UK that shares a land border with another sovereign state—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 United Kingdom is a unitary state governed under a constitutional monarchy and a parliamentary system, with its seat of government in the capital city of London. It is a country in its own right and consists of four countries: England, Northern Ireland, Scotland and Wales. There are three devolved administrations, each with varying powers, based in Belfast, Edinburgh and Cardiff, the capitals of Northern Ireland, Scotland, and Wales. Associated with the UK, but not constitutionally part of it, are three Crown Dependencies. The United Kingdom has fourteen overseas territories. These are remnants of the British Empire which, at its height in 1922, encompassed almost a quarter of the world's land surface and was the largest empire in history. British influence can still be observed in the language, culture and legal systems of many of its former territories.

Great Britain or Britain (Welsh: Prydain Fawr, Scottish Gaelic: Breatainn Mhòr, Cornish: Breten Veur) is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, the largest European island, and the largest of the British Isles. With a population of about 60.0 million people in mid-2009, it is the third most populous island in the world, after Java and Honshū. Great Britain is surrounded by over 1,000 smaller islands and islets. The island of Ireland lies to its west. Politically, Great Britain may also refer to the island itself together with a number of surrounding islands which comprise the territory of England, Scotland and Wales.

All of the island is territory of the sovereign state of the United Kingdom of Great Britain and Northern Ireland, and most of the United Kingdom's territory is in Great Britain. Most of England, Scotland, and Wales are on the island of Great Britain, as are their respective capital cities: London, Edinburgh, and Cardiff.

Northern Ireland (Irish: Tuaisceart Éireann pronounced [ˈt̪ˠuəʃcəɾˠt̪ˠ ˈeːɾʲən̪ˠ] ( listen), Ulster Scots: Norlin Airlann or Norlin Airlan) is a part of the United Kingdom in the north-east of the island of Ireland. It shares a border with the Republic of Ireland to the south and west. As of 2001, its population was 1,685,000, constituting about 30% of the island's total population and about 3% of the population of the United Kingdom. Since the signing of the Good Friday Agreement in 1998, Northern Ireland is largely self-governing. According to the agreement, Northern Ireland co-operates with the rest of Ireland – from which it was partitioned in 1921 — on some policy areas, while other areas are reserved for the Government of the United Kingdom, though the Republic of Ireland "may put forward views and proposals".

Northern Ireland was for many years the site of a violent and bitter inter-communal conflict — the Troubles — which was caused by divisions between nationalists, who are predominantly Roman Catholic, and unionists, who are predominantly Protestant. Unionists want Northern Ireland to remain as a part of the United Kingdom, and generally see themselves as British, while nationalists wish for it to be politically reunited with the rest of Ireland, independent of British rule, and generally see themselves as Irish. Additionally, people from both sides of the community may describe themselves as Northern Irish. Since 1998, most of the paramilitary groups involved in the Troubles have ceased their armed campaigns.