a kingdom of NW Europe, consisting chiefly of the island of Great Britain together with Northern Ireland: became the world's leading colonial power in the 18th century; the first country to undergo the Industrial Revolution. It became the United Kingdom of Great Britain and Northern Ireland in 1921, after the rest of Ireland became autonomous as the Irish Free State. Primarily it is a trading nation, the chief export is services and London is the world's largest financial centre; joined the Common Market (now the European Union) in January 1973; referendum (2016) decided narrowly in favour of the UK leaving the European Union, which it did in 2020. Official language: English; Gaelic, Welsh, and other minority languages. Religion: Christian majority. Currency: pound sterling. Capital: London. Pop: 66 181 585 (2017 est). Area: 244 110 sq km (94 251 sq miles)