The United Kingdom of Great Britain and Northern Ireland i É¡reɪt Ëbrɪtn Énd ËnÉËðn ËÊɪÉlÉnd , commonly known as the United Kingdom or Britain, is a sovereign state in Europe. Lying off the north-western coast of the European mainland, the country includes the island of Great Britain , the north-eastern part of the island of Ireland, and many smaller islands. Northern Ireland is the only part of the UK that shares a land border with another state: the Republic of Ireland. Apart from this land border, the UK is surrounded by the Atlantic Ocean, with the North Sea in the east and the English Channel in the south. The Irish Sea lies between Great Britain and Ireland. The UK has an area of 243,610 square kilometres , making it the 78th-largest sovereign state in the world and the 11th-largest in Europe.