Union Flag or Union Jack: Which is correct?

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

Union Jack United Kingdom Flag Vector Images - FreePatternsArea

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First Flag of the United States

👉 Union Jack flag template, Activities

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

Union Jack flag of the United Kingdom 10870759 PNG

Union Jack: Do you know the correct way up?

Union Jack specfications • Piggotts Flags and Branding

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

What is the difference between Union Jack and flag?

Union Jack - Wikipedia

The Flags That Make Up the Union Jack – Patriot Wood

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