The terms Union Flag and Union Jack are both historically correct for describing the de facto national flag of the United Kingdom. Whether to call it the "Union Flag" or the "Union Jack" is a matter of debate by many. According to the Flag Institute, the vexillological organisation for the United Kingdom, "the national flag of the United Kingdom, the Crown Dependencies and Overseas Territories is the Union Flag, which may also be called the Union Jack."[4] The Institute also notes:

It is often stated that the Union Flag should only be described as the Union Jack when flown in the bows of a warship, but this is a relatively recent idea. 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. In 1908, a government minister stated, in response to a Parliamentary question, that "the Union Jack should be regarded as the National flag".[5][6]

Nevertheless, the term "Union Flag" is used in King Charles's 1634 proclamation:

... none of Our Subjects, of any of Our Nations and Kingdoms shall from henceforth presume to carry the Union Flag in the Main top, or other part of any of their Ships (that is) St Georges cross and St Andrew's Cross joined together upon pain of Our high displeasure, but that the same Union Flag be still reserved as an ornament proper for Our own Ships and Ships in Our immediate Service and Pay, and none other."[7]
—Proclamation appointing the Flag, as well for Our Navy Royal as for the Ships of Our Subjects of South and North Britain – 5 May 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:

And that the Union Flag shall be Azure, the Crosses Saltires of St. Andrew and St. Patrick Quarterly per Saltire, counterchanged Argent and Gules; the latter fimbriated of the Second, surmounted by the Cross of St. George of the Third, fimbriated as the Saltire : ...[8]
—A Proclamation Declaring His Majesty's Pleasure concerning the Royal Style and Titles appertaining to the Imperial Crown of the United Kingdom of Great Britain and Ireland, and its Dependencies, and also the Ensigns, Armorial Flags, and Banners thereof"

When the first flag representing Britain was introduced on the proclamation of King James I in 1606,[9] it became known simply as "the British flag" or "the flag of Britain". The royal proclamation gave no distinctive name to the new flag. The word "jack" was in use before 1600 to describe the maritime bow flag.[5] 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.[3]

Amongst the proclamations issued by King George III at the time of the Union of 1801 was a proclamation concerning flags at sea, which referred to "Our Flags, Jacks, and Pendants" and forbade merchant vessels from wearing "Our Jack, commonly called the Union Jack" nor any pendants or colours used by the King's ships.[10] In contrast, the King's proclamation of the same day concerning the arms and flag of the United Kingdom, not colours at sea, called the new flag "the Union Flag".[citation needed]

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 colonies, it is possible that the term "Jack" occurred because of its regular use on all British ships using the "Jack Staff" (a flag pole attached to the bow of a ship). The term 'Union Jack' possibly dates from Queen Anne's time (r. 1702–14), but its origin is uncertain. It may come from the 'jack-et' of the English or Scottish soldiers, or from the name of James I who originated the first union in 1603. Even if the term "Union Jack" does derive from the jack flag, after three centuries, it is now sanctioned by use, has appeared in official use, confirmed as the national flag by Parliament and remains the popular term.[11]

the jack staff as I recall the the small mast on the bow of a ship where the union flag, was the only flag allowed. when flown from the mast was referred to as the battle flag when engaging an enemy, strictly speaking the union jack, was not the flag but but the foremast that's only purpose was to fly a flag, flags very rarely fluttered to the stern as were driven by the same wind that powered the ship, nowadays engine powered ships travel more often than not, faster than the prevailing wind

RN Warships fly the White Ensign at the Stern Jim..

yes I do know that, the union flag is only flown on special occasions, same as when entering a foreign port a flag of that nation is also flown as a courtesy. the white ensign is flown from the mast head when engaging an enemy another strange custom is ships flying the red ensign, usually lower and raise it when passing a warship.