The United Kingdom of Great Britain and Northern Ireland[note 5] (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[10][11] and consists of four countries: England, Northern Ireland, Scotland and Wales.[12] There are three devolved national administrations, each with varying powers,[13][14] situated in Belfast, Cardiff and Edinburgh; the capitals of Northern Ireland, Wales and Scotland respectively. Associated with the UK, but not constitutionally part of it, are three Crown Dependencies.[15] The United Kingdom has fourteen overseas territories.[16] 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.
The UK is a developed country and has the world's sixth-largest economy by nominal GDP and seventh-largest economy by purchasing power parity. It was the world's first industrialised country[17] and the world's foremost power during the 19th and early 20th centuries.[18] The UK remains a great power with leading economic, cultural, military, scientific and political influence.[19] It is a recognised nuclear weapons state and its military expenditure ranks third or fourth in the world.[20] The UK has been a permanent member of the United Nations Security Council since its first session in 1946; it is also a member of the Commonwealth of Nations, the Council of Europe, the European Union, the G7, the G8, the G20, NATO, the OECD and the World Trade Organization.
大不列颠及北爱尔兰联合王国（英语：The United Kingdom of Great Britain and Northern Ireland；威尔斯语：Teyrnas Unedig Prydain Fawra Gogledd Iwerddon；蘇格兰盖尔语：An Rìoghachd Aonaichte na Breatainn Mhòr agus Eirinn mu Thuath；爱尔兰语：Ríocht Aontaithe na Breataine Móire agus Thuaisceart Éireann；低地蘇格兰语：Unitit Kingdom o Great Britain an Norlin Airlann），简称联合王国（英文：United Kingdom）或不列颠（英文：Britain）[2]，通称英国，是由大不列颠岛上的英格兰、蘇格兰和威尔斯，以及爱尔兰岛东北部的北爱尔兰以及一系列附属岛屿共同组成的一个西欧岛国。中文里的「英国」一词，即由「英格兰」而来，其国际代码为GB。
英国本土位於欧洲大陆西北面的不列颠群岛，被北海、英吉利海峡、凯尔特海、爱尔兰海和大西洋包围。除了英国本土之外，还包括十四个海外领地[3]。
联合王国由四个国家联合而成，分别是英格兰、蘇格兰、北爱尔兰和威尔斯[4]。英国采用议会制政体，王国政府所在地为伦敦，但是同时拥有其他三个国家级行政机构分别位於贝尔法斯特（北爱尔兰），卡地夫（威尔斯）和爱丁堡（蘇格兰）。英国是世界上第一个君主立宪制国家，现任君主是伊丽莎白二世女王。
英王国在最强盛时期连同其海外殖民地被称为大英帝国，该帝国在1922年的时候达到巅峰，拥有全世界四分之一的陆地，是有史以来世界上面积最大的国家。大英帝国解体後，英国继续在语言，文化，政治上对其前殖民地保留了巨大的影响力，英国女王伊丽莎白二世现在仍然是大英国协组织的首脑，和其他15个大英国协国家的国家元首。
英国是一个已开发国家、世界上第一个工业化国家[5]，在19世纪和20世纪早期是世界上最强大的国家[6]，但是经过两次世界大战和20世纪下半叶大英帝国的崩溃，英国已经失去了曾经在国际事务上的领导力。不过，英国仍然是一个在世界范围内拥有巨大影响力、举足轻重的政治、经济、文化和军事强国。
英国是第一、第二次世界大战主要战胜国，拥有核武器和世界上第三的军费开支，是联合国安理会常任理事国，对议案拥有否决权。英国亦为欧盟成员国、北约创始会员国、八大工业国组织成员国之一和欧洲四大经济体之一、全球前七大经济体、也是一系列国际组织如经合组织、世界贸易组织的一员。
