When the 'Union Jack' was first introduced in 1606, it was known simply as 'the British flag' or 'the flag of Britain', and was ordered to be flown at the main masthead of all ships, warships and merchant ships, of both England and Scotland. 

The first use of the name 'Union' appears in 1625. There are various theories as to how it became known as the 'Union Jack' but most of the evidence points to the name being derived from the use of the word 'jack' as a diminutive. This word was is use before 1600 to describe a small flag flown from the small mast mounted on the bowsprit, and by 1627 it appears that a small version of the Union flag was commonly flown in this position. For some years it was 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.