Union Flag or Union Jack: Which is correct?

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

Scottish independence: which new British flag did you choose in

3x5ft British Flag / UK Flag / Union Jack

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Flag - Union Jack, World War II Victory in Europe (VE) Day, 1945

How is The Queen's Union Flag made?

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British Flag - 5 Facts we bet you didn't know - U.K.ABROAD

Jeff Bridgman Antique Flags and Painted Furniture - BRITISH UNION

Union Jack: Do you know the correct way up?

How to know if a Union Jack has been hung upside down - Quora

British flag or Union Jack. A vertically hanging flag representing

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Is the Irish flag referenced anywhere in the Union Jack? - Quora

Without Scotland, What Becomes of the Union Jack?

Why is the United Kingdom flag called the Union Jack? - Great