The British flag
The Union Flag (known as the Union Jack ) is the national flag of the United Kingdom.

The flag was first proclaimed by James I (James VI of Scotland) in 1606 when he inherited both the thrones of England and Scotland and sought to create a flag combining the two crosses of the respective nations. Ireland was later added in 1801 when it joined the United Kingdom.
It’s Actually Three Flags in One
The flag is actually made up of the three flags of England, Scotland and Ireland which are the crosses of each country’s patron saint.
England: Cross of St George –Red Cross

Scotland: Cross of St Andrew – White Saltaire

Ireland: Cross of St Patrick – Red Saltaire

Wales is Missing

While there are four constituent countries that make up the United Kingdom, only three of them are actually represented on the Union Flag. That’s because technically, Wales is a principality and has legally always been considered a part of England .
There are 23 countries using the Union Flag today.
There are still 23 countries around the world – many small territories or islands – that use some form of the Union Flag in their design. But there are big ones that still do like Australia and New Zealand.
Historian David Starkey said  that the Union Flag is called 'Jack' because it is named after James l of Great Britain (Jacobus, Latin for James ),who introduced the flag following his accession to the throne.