Union Flag or Union Jack: Which is correct?

In which situations should the terms 'Union Jack' and 'Union Flag' be used?

UK flag protocol

Who Can Fly The Red Ensign Flag?

British flag hi-res stock photography and images - Alamy

File:Flags of the Union Jack.svg - Wikimedia Commons

The Great Britain Union Flag comes printed on all weather 200 denier Nylon and is made in the USA. These flags are made to order. This is the official

Great Britain Union Flag (1707-1800)

How is The Queen's Union Flag made?

TIL Upside-down Union Jack : r/CasualUK

UK media speculates on future of British flag without Scotland

Search Forces Network

Union Jack United Kingdom Flag Vector Images - FreePatternsArea

British Red Ensign Flag for Sale – Colonial Flag

British Flag Svg,distressed UK Flag Svg,union Jack Svg,uk Flag Svg