The most plausible one is that when a small flag was mounted on the front of a warship (and a selected number of other ships) it was called 'the Jack'. Sometime around 1674 the British flag became formally known as the 'Union Jack' when mounted on a warship and the ship was not in harbour.

Why is the Union Jack now called the Union flag?

The Union Flag, or Union Jack, is the national flag of the United Kingdom. It is so called because it combines the crosses of the three countries united under one Sovereign - the kingdoms of England and Wales, of Scotland and of Ireland (although since 1921 only Northern Ireland has been part of the United Kingdom).

What is the correct Union Flag?

The Union Flag must be flown the correct way up. This is with the wider diagonal white stripe above the red diagonal stripe in the half nearest to the flag pole. The wider diagonal white stripe should be above the red diagonal stripe at the top left hand side of the Flag nearest the flag pole.