With an area of 242,500 square kilometres (93,600 sq mi), the UK is the 78th-largest sovereign state in the world.
It is also the 21st-most populous country, with an estimated 65.5 million inhabitants in 2016.
The Roman conquest, beginning in 43 AD, and the 400-year rule of southern Britain, was followed by an invasion by Germanic Anglo-Saxon settlers, reducing the Brythonic area mainly to what was to become Wales, Cornwall and the historic Kingdom of Strathclyde.
The adjective "British" is commonly used to refer to matters relating to the United Kingdom.
The term has no definite legal connotation, but is used in law to refer to United Kingdom citizenship and matters to do with nationality.
The UK consists of four countries—England, Scotland, Wales and Northern Ireland.
based in their capitals Edinburgh, Cardiff and Belfast, respectively.
The UK is also a member of the Commonwealth of Nations, the Council of Europe, the G7 finance ministers, the G7 forum, the G20, NATO, the Organisation for Economic Co-operation and Development (OECD), and the World Trade Organization (WTO).
The 1707 Acts of Union declared that the kingdoms of England and Scotland were "United into One Kingdom by the Name of Great Britain", though the new state is also referred to in the Acts as the "Kingdom of Great Britain", "United Kingdom of Great Britain" and "United Kingdom".This meant that the United Kingdom created in 1707 by the Treaty of Union between England and Scotland encompassed all of Great Britain.The Kingdom of Ireland merged with this state in 1801 to form the United Kingdom of Great Britain and Ireland.Apart from this land border, the UK is surrounded by the Atlantic Ocean, with the North Sea to its east, the English Channel to its south and the Celtic Sea to its south-south-west, giving it the 12th-longest coastline in the world.The Irish Sea lies between Great Britain and Ireland.The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a sovereign country in western Europe.