Union Flag or Union Jack: Which is correct?

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

Search Forces Network

There's something wrong with this Union Jack – but most Brits can

Jeff Bridgman Antique Flags and Painted Furniture - BRITISH UNION

How is The Queen's Union Flag made?

WWI Union Jack Silk Flag, Circa 1918 - The Great Republic

How can one tell if the British National Flag, the 'Union Jack

United Kingdom Great Britain 4in x 6in Polyester Flag

Search Forces Network

Majority of Brits can't tell if the UK flag is being flown upside

The Iconic Artistic History of the Union Flag – Enter Gallery

Maritime flags of Great Britain — Naval Action

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)

Will This Be the U.K.'s New Flag? - The Atlantic

Union Jack: Do you know the correct way up? – RNZN Communicators Association