BRITAIN, the largest island in the archipelago just north of France; the island of Britain contains three countries:
ENGLAND, SCOTLAND, and WALES.
ENGLAND is one of the countries of Britain.
SCOTLAND is one of the countries of Britain.
WALES is one of the countries of Britain.
NORTHERN IRELAND is a jurisdiction having approximately the same status as England, Scotland, and Wales, but on a different island.
The UNITED KINGDOM is the union of England, Scotland, Wales, and Northern Ireland, including whatever islands are also included in those countries. The full name of the United Kingdom is THE UNITED KINGDOM OF GREAT BRITAIN AND NORTHERN IRELAND. The United Kingdom itself is a country. Thus it is a country that is made up of four countries. A country made of countries might seem a paradox, yet the countries that make up the UK, especially England, Scotland, and Wales, do not think of themselves as anything less; Scotland has its own Parliament and banknotes, Wales has its own language and National Assembly, all three have national identities going back more than a thousand years, and the Encyclopedia Britannica calls them countries (next item). Perhaps more to the point, ENGLAND, SCOTLAND, WALES, and NORTHERN IRELAND are listed in the USPS Index of Countries and Localities. (According to the Oxford English Dictionary, the term UNITED KINGDOM was first used in 1801 and, until 1921, included Ireland.)