The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Northwestern Europe, off the coast of the continental mainland. It comprises England, Scotland, Wales and Northern Ireland, with a population of over 69 million in 2024. 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). It shares a land border with the Republic of Ireland and is otherwise surrounded by the Atlantic Ocean, the North Sea, the English Channel, the Celtic Sea and the Irish Sea, while maintaining sovereignty over the British Overseas Territories. The capital and largest city of both England and the UK is London; Edinburgh, Cardiff and Belfast are the national capitals of Scotland, Wales and Northern Ireland.

Britain has been inhabited 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 flourish, 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 with three distinct jurisdictions: England and Wales, Scotland, and Northern Ireland. 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. As a nuclear state with one of the highest defence budgets, the UK maintains one of the strongest militaries in Europe. 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. A great power, the UK is part of numerous international organisations and forums.

The Acts of Union 1707 declared that the Kingdom of England and the Kingdom of Scotland were "United into One Kingdom by the Name of Great Britain". 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". 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".

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

The term "Great Britain" conventionally refers to the island of Great Britain, or politically to England, Scotland and Wales in combination. It is sometimes used as a loose synonym for the United Kingdom as a whole. 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.

The term "Britain" is used as a synonym for Great Britain, but also sometimes for the United Kingdom. Usage is mixed: the UK Government style guide prefers the term "UK" rather than "Britain" or "British" (except when referring to embassies), while other government documents acknowledge that both terms refer to the United Kingdom and that elsewhere "British government" is used at least as frequently as "United Kingdom government". The UK Permanent Committee on Geographical Names recognises "United Kingdom", "UK" and "U.K." as shortened and abbreviated geopolitical terms for the United Kingdom of Great Britain and Northern Ireland in its toponymic guidelines; it does not list "Britain" but notes that "it is only the one specific nominal term 'Great Britain' which invariably excludes Northern Ireland". The BBC historically preferred to use "Britain" as shorthand only for Great Britain, though the present style guide does not take a position except that "Great Britain" excludes Northern Ireland.