The United Kingdom is a country.
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 . The United Kingdom is a developed country and has the world 's fifth-largest economy by nominal GDP and ninth-largest economy by purchasing power parity . It was the world 's first industrialised country and the world 's foremost power during the 19th and early 20th centuries .