The Union Flag, popularly known as the Union Jack, is the national flag of the United Kingdom. It is the British flag representing all of the UK.
It is called the Union Flag because it symbolises the administrative union of the countries of the United Kingdom. It is made up of the individual Flags of three of the Kingdom's countries all united under one Sovereign - the countries of England, Scotland and of Northern Ireland (since 1921 only Northern Ireland has been part of the United Kingdom). As Wales was not a Kingdom but a Principality it could not be included on the flag.
By the way, 'Union Jack' was officially acknowledged as an alternative name for the Union Flag by the Admiralty and Parliament in the early 20th century. The term 'jack' refers to the flag that is flown from the bowsprit of a ship, often denoting nationality.
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