Why is the union flag called "Union Jack"?英国国旗为什么叫Union Jack英语回答要求字数在100词以内

Whether to use "Union Flag" or "Union Jack" is a matter of debate. One view is that "Union Jack" should be used only for the flag when it is flown as a jack (a small flag flown at the bow of a ship), but it is not universally accepted that the "Jack" of "Union Jack" is a reference to such a jack flag and is only an educated guess.
The Flag Institute, the vexillological organisation for the United Kingdom, stated that the term Union Flag is a "relatively recent idea". It also noted that "From early in its life the Admiralty itself frequently referred to the flag as the Union Jack, whatever its use, and in 1902 an Admiralty Circular announced that Their Lordships had decided that either name could be used officially. Such use was given Parliamentary approval in 1908 when it was stated that "the Union Jack should be regarded as the National flag". Nevertheless, the term "Union Flag" is used in King James's proclamations of 1606 and 1634, and in King George III's proclamation of 1 January 1801 concerning the arms and flag of the United Kingdom of Great Britain and Ireland.
When the first flag was introduced in 1606, it became known simply as "the British flag" or "the flag of Britain", although the royal proclamation had called it "the Union Flag". The word 'jack' was in use before 1600 to describe the maritime bow flag[citation needed]. By 1627 a small Union Jack was commonly flown in this position. One theory goes that for some years it would have been called just "the Jack", or "Jack flag", or "the King's Jack", but by 1674, while formally referred to as "His Majesty's Jack", it was commonly called the Union Jack, and this was officially acknowledged.
The size and power of the Royal Navy internationally at the time could also explain why the flag was named the "Union Jack"; considering the navy was so widely utilised and renowned by the United Kingdom and Commonwealth countries, it is possible that the term "Jack" did occur because of its regular use on all British ships using the "Jack Staff" (a flag pole attached to the bow of a ship). Even if the term "Union Jack" does derive from the jack flag (as perhaps seems most likely), after three centuries, it is now sanctioned by use, has appeared in official use, and remains the popular term.
The BBC website does not use the term "union flag" because of its "great potential for confusion", preferring union jack (in lower case). The Merchant Shipping Act 1995 refers to the national colours of the United Kingdom as "the Union flag (commonly known as the Union Jack)".

"Jack"in the dictionary also means "ship's flag flown to show nationality".
Because Northern Ireland,England,Wales and Scotland joined together to become the United Kindom and this was shown to the world in a new flag called the Union Jack.Besides,the Union Jack was designed based on the flags of the four countries.All the crossing on the four flags were joined into the Union Jack.You can find either flag of the four countries in the Union Jack,so to speak.
Now you can see why the Union Jack is called "the Union Jack".