The terms United Kingdom (UK), Britain, Great Britain, England, and the British Isles are often used, albeit incorrectly, as synonyms. The “UK” is an abbreviation used for the sovereign state, United Kingdom of Great Britain and Northern Ireland. It is a country made up of four culturally distinct countries – England, Scotland, Wales, and Northern Ireland (which includes only some part of Ireland; the rest of Ireland — Republic of Ireland — is a separate sovereign country).
By 925 AD, the Anglo-Saxon tribes across England had all been united and the Kingdom of England had been formed. It was later, through the Acts of Union of 1536 and 1542 that Wales united with the Kingdom of England. King Henry VII united the country and enacted uniform laws across the Kingdom of England and Wales.
The King of Scots, James VI, inherited the thrones of England, Scotland, and Ireland in 1603 and though the kingdoms of Scotland and England were sovereign states with separate parliaments and laws, they were united by sharing a common monarch. Described as a union of crowns, it simply meant that the same person wore both crowns. The Acts of Union formally joined the two kingdoms – “United into One Kingdom by the Name of Great Britain”. The Union with Scotland Act was passed by the Parliament of England in 1706. The following year, the Union with England Act was passed by the Parliament of Scotland.
A similar Act of Union was passed in 1800 when Ireland formally joined Great Britain, to form the United Kingdom of Great Britain and Ireland.
Technically, Great Britain is divided into three countries that are part of the United Kingdom: England, Scotland, and Wales. The word ‘United Kingdom’ came into existence only after the Act of Union formally joined Ireland and Great Britain, as explained earlier.
Following the Irish War of Independence, the Irish Republic seceded from the union in 1922, leaving only Northern Ireland as part of the United Kingdom of Great Britain and Northern Ireland.
The British Isles is a term used to describe the archipelago (of about 6,000 islands) that includes both the UK and the Republic of Ireland as well. Great Britain is the largest of these.
Another term that is commonly misunderstood is the Commonwealth of Nations. The Commonwealth is an association of countries that are erstwhile British colonies. The Commonwealth nations are all politically independent countries now.