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.大不列顛及北愛(ài)爾蘭聯(lián)合王國(guó)（英語(yǔ)：The United Kingdom of Great Britain and Northern Ireland；威爾斯語(yǔ)：Teyrnas Unedig Prydain Fawra Gogledd Iwerddon；蘇格蘭蓋爾語(yǔ)：An Rìoghachd Aonaichte na Breatainn Mhòr agus Eirinn mu Thuath；愛(ài)爾蘭語(yǔ)：Ríocht Aontaithe na Breataine Móire agus Thuaisceart éireann；低地蘇格蘭語(yǔ)：Unitit Kingdom o Great Britain an Norlin Airlann），簡(jiǎn)稱(chēng)聯(lián)合王國(guó)（英文：United Kingdom）或不列顛（英文：Britain）[2]，通稱(chēng)英國(guó)，是由大不列顛島上的英格蘭、蘇格蘭和威爾斯，以及愛(ài)爾蘭島東北部的北愛(ài)爾蘭以及一系列附屬島嶼共同組成的一個(gè)西歐島國(guó)。中文里的「英國(guó)」一詞，即由「英格蘭」而來(lái)，其國(guó)際代碼為GB。英國(guó)本土位於歐洲大陸西北面的不列顛群島，被北海、英吉利海峽、凱爾特海、愛(ài)爾蘭海和大西洋包圍。除了英國(guó)本土之外，還包括十四個(gè)海外領(lǐng)地[3]。聯(lián)合王國(guó)由四個(gè)國(guó)家聯(lián)合而成，分別是英格蘭、蘇格蘭、北愛(ài)爾蘭和威爾斯[4]。英國(guó)采用議會(huì)制政體，王國(guó)政府所在地為倫敦，但是同時(shí)擁有其他三個(gè)國(guó)家級(jí)行政機(jī)構(gòu)分別位於貝爾法斯特（北愛(ài)爾蘭），卡地夫（威爾斯）和愛(ài)丁堡（蘇格蘭）。英國(guó)是世界上第一個(gè)君主立憲制國(guó)家，現(xiàn)任君主是伊麗莎白二世女王。英王國(guó)在最強(qiáng)盛時(shí)期連同其海外殖民地被稱(chēng)為大英帝國(guó)，該帝國(guó)在1922年的時(shí)候達(dá)到巔峰，擁有全世界四分之一的陸地，是有史以來(lái)世界上面積最大的國(guó)家。大英帝國(guó)解體後，英國(guó)繼續(xù)在語(yǔ)言，文化，政治上對(duì)其前殖民地保留了巨大的影響力，英國(guó)女王伊麗莎白二世現(xiàn)在仍然是大英國(guó)協(xié)組織的首腦，和其他15個(gè)大英國(guó)協(xié)國(guó)家的國(guó)家元首。英國(guó)是一個(gè)已開(kāi)發(fā)國(guó)家、世界上第一個(gè)工業(yè)化國(guó)家[5]，在19世紀(jì)和20世紀(jì)早期是世界上最強(qiáng)大的國(guó)家[6]，但是經(jīng)過(guò)兩次世界大戰(zhàn)和20世紀(jì)下半葉大英帝國(guó)的崩潰，英國(guó)已經(jīng)失去了曾經(jīng)在國(guó)際事務(wù)上的領(lǐng)導(dǎo)力。不過(guò)，英國(guó)仍然是一個(gè)在世界范圍內(nèi)擁有巨大影響力、舉足輕重的政治、經(jīng)濟(jì)、文化和軍事強(qiáng)國(guó)。英國(guó)是第一、第二次世界大戰(zhàn)主要戰(zhàn)勝?lài)?guó)，擁有核武器和世界上第三的軍費(fèi)開(kāi)支，是聯(lián)合國(guó)安理會(huì)常任理事國(guó)，對(duì)議案擁有否決權(quán)。英國(guó)亦為歐盟成員國(guó)、北約創(chuàng)始會(huì)員國(guó)、八大工業(yè)國(guó)組織成員國(guó)之一和歐洲四大經(jīng)濟(jì)體之一、全球前七大經(jīng)濟(jì)體、也是一系列國(guó)際組織如經(jīng)合組織、世界貿(mào)易組織的一員。