Union Flag or Union Jack: Which is correct?

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

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Union Flag or Union Jack: Which is correct?

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

Free Printable Union Jack Flag - Crafts on Sea

united kingdom - If the Union Jack joins the flag of England and

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Union Jack: Do you know the correct way up?

Union Jack - British Flag - Brush Style Digital Art by Mister Tee

Union Jack Flags for Sale, Made In The UK, Flagmakers

Curious Questions: How can you tell if the Union Jack is upside