by 
The Curious Bag
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The British flag has been known as the Union Jack since at least 1674, and its etymology stems from the union of earlier kingdoms and the predominance of the navy in British society.
Stuarts Unite Britain
The Royal House of Stuart held claims to the thrones of Scotland, England and Ireland when each country was independent, thus creating a union that King James I described as the Kingdom of Great Britain.
What's a Jack?
Jack is the slang term for a naval flag, or ensign, and was first used to describe a flag flown from the bowsprit of a ship.
Formal vs. Informal
The jack for the union of Great Britain was officially named "His Majesty's Jack" in 1674, but was commonly referred to as the Union Jack, and was officially recognized by that name as well.
Navy vs. Civilian
The Union Jack can be flown only by British navy vessels; civilians can fly a flag similar to it but bordered with a white outline.
Union Flag?
Some consider the flag to be a "jack" only when flown from a ship, but Parliament decreed in 1908 that "the Union Jack should be regarded as the National flag."
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