The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,^[m] is a country in Northwestern Europe, off the coast of the continental mainland. It comprises England, Scotland, Wales and Northern Ireland.^[n] The UK includes the island of Great Britain, the north-eastern part of the island of Ireland, and most of the smaller islands within the British Isles, covering 94,354 square miles (244,376 km²).^[f] Northern Ireland shares a land border with the Republic of Ireland; otherwise, the UK is surrounded by the Atlantic Ocean, the North Sea, the English Channel, the Celtic Sea and the Irish Sea. It maintains sovereignty over the British Overseas Territories, which are located across various oceans and seas globally. The UK had an estimated population of over 68.2 million people in 2023. The capital and largest city of both England and the UK is London. The cities of Edinburgh, Cardiff and Belfast are the national capitals of Scotland, Wales and Northern Ireland.

The UK has been inhabited continuously since the Neolithic. In AD 43 the Roman conquest of Britain began; the Roman departure was followed by Anglo-Saxon settlement. In 1066 the Normans conquered England. With the end of the Wars of the Roses the Kingdom of England stabilised and began to grow in power, resulting by the 16th century in the annexation of Wales and the establishment of the British Empire. Over the course of the 17th century the role of the British monarchy was reduced, particularly as a result of the English Civil War. In 1707 the Kingdom of England and the Kingdom of Scotland united under the Treaty of Union to create the Kingdom of Great Britain. In the Georgian era the office of prime minister became established. The Acts of Union 1800 incorporated the Kingdom of Ireland to create the United Kingdom of Great Britain and Ireland in 1801. Most of Ireland seceded from the UK in 1922 as the Irish Free State, and the Royal and Parliamentary Titles Act 1927 created the present United Kingdom.

The UK became the first industrialised country and was the world’s foremost power for the majority of the 19th and early 20th centuries, particularly during the Pax Britannica between 1815 and 1914. The British Empire was the leading economic power for most of the 19th century, a position supported by its agricultural prosperity, its role as a dominant trading nation, a massive industrial capacity, significant technological achievements, and the rise of 19th-century London as the world’s principal financial centre. At its height in the 1920s the empire encompassed almost a quarter of the world’s landmass and population, and was the largest empire in history. However, its involvement in the First World War and the Second World War damaged Britain’s economic power, and a global wave of decolonisation led to the independence of most British colonies.

The UK is a constitutional monarchy and parliamentary democracy.^[o] It has three distinct jurisdictions: England and Wales, Scotland, and Northern Ireland. Since 1999 Scotland, Wales and Northern Ireland have their own governments and parliaments which control various devolved matters. A developed country with an advanced economy, the UK ranks amongst the largest economies by nominal GDP, and is one of the world’s largest exporters and importers. It is a nuclear state with one of the world’s highest military budgets. Its soft power influence can be observed in the legal and political systems of many of its former colonies, and British culture remains globally influential, particularly in language, literature, music and sport. English is the world’s most-widely-spoken language and the third-most-spoken native language. The UK is part of multiple international organisations and forums.

The Acts of Union 1707 declared that the Kingdom of England and Kingdom of Scotland were “United into One Kingdom by the Name of Great Britain”.^[p][22] The term “United Kingdom” has occasionally been used as a description for the former Kingdom of Great Britain, although its official name from 1707 to 1800 was simply “Great Britain”.^[23] The Acts of Union 1800 formed the United Kingdom of Great Britain and Ireland. Following the partition of Ireland and the independence of the Irish Free State in 1922, which left Northern Ireland as the only part of the island of Ireland within the United Kingdom, the name was changed in 1927 to the “United Kingdom of Great Britain and Northern Ireland”.^[24]

Although the United Kingdom is a sovereign country, England, Scotland, Wales and Northern Ireland are also widely referred to as countries.^[25] The UK Prime Minister’s website has used the phrase “countries within a country” to describe it.^[26] Some statistical summaries, such as those for the twelve NUTS 1 regions, refer to Scotland, Wales and Northern Ireland as “regions”.^[27] Northern Ireland is also referred to as a “province”.^[28] With regard to Northern Ireland, the descriptive name used “can be controversial, with the choice often revealing one’s political preferences”.^[29]

The term “Great Britain” conventionally refers to the island of Great Britain, or politically to England, Scotland and Wales in combination.^[30] It is sometimes used as a loose synonym for the United Kingdom as a whole.^[31] The word England is occasionally used incorrectly to refer to the United Kingdom as a whole, a mistake principally made by people from outside the UK.^[32]