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 . It is also the 21st-most populous country , with an estimated 65.1 million inhabitants . Together , this makes it the fourth-most densely populated country in the European Union ( EU ) . It was the world 's first industrialised country and the world 's foremost power during the 19th and early 20th centuries .