Why is the British flag called the ‘Union Jack’?

The Union Jack, the official flag of Britain since 1801, is really three separate flags in one, for it combines the English cross of St George, the Scottish cross of St Andrew and the Irish cross of St Patrick. England, Scotland and Ireland were originally independent countries, and the Union Jack symbolizes the fact that they now from the United Kingdom, another name for Britain. Although generally known as the Union Jack, this is incorrect because it is the term for a flag flown only from the jack staff in the bows of a ship to indicate it is a man-of-war, through popular use, however, the Union Jack has become the accepted name for the flag.