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 . The capital of the United Kingdom and its largest city is London , a global city and financial centre with an urban area population of 10.3 million , the fourth-largest in Europe and second-largest in the European Union .