The Union Flag..

[Mattnall]

Either term is correct it appears.

According to the Flag Institute:

From its earliest days, the Admiralty often referred to the flag – however it was used – as the Union Jack. In 1902 an Admiralty Circular announced that either name could be used officially. And in 1908 the UK Parliament approved this verdict, stating that 'the Union Jack should be regarded as the National flag'.

It was always known as the Union Flag unless it's flown from the jackstaff, in which case it's the Union Jack.

Yes, as I understand it the jackstaff got its name because the Union Jack flew from it.
Charles II said all navy ships had to fly a small (or jack) version of the flag.

[Dark Skies]

I thought everyone knew the difference - certainly if you're a navy brat like me.

[1066]

I thought everyone knew the difference - certainly if you're a navy brat like me.

I'm not too nitpicky about what people call the flag but it does irk me when it's flown upside down. If you want to show your patriotic enough to fly the flag you should at least know which way to fly it. Most flags flown from government buildings, town halls etc. the flag has a toggle attachment so even millennial airheads can't get it wrong. There's not much you can do about the multitude that are wrong in peoples gardens but where those on more public display like businesses, shopping centres and the like should be made aware of their error.

I find an email pointing out the error of their ways usually does the trick.

[Dark Skies]

I thought everyone knew the difference - certainly if you're a navy brat like me.

I'm not too nitpicky about what people call the flag but it does irk me when it's flown upside down. ...

This was a constant irritation for my late father (Cdr RN) who made the mistake of buying a property by the Thames in Oxford.
"Look at those Sunday sailors! A flotilla of f***wits !!!"
A walk with my father along the river would often be punctuated by him shouting out to some Captain Pugwash to strike their flag and hoist it the right way up.
That and shouting at boaters to pull their fenders in whenever he saw them trailing in the water and causing excessive wash. It often led to angry words. It embarrassed my mum.
But he was right, the excessive wash from clowns in boats dragging their fenders damages the banks. Not to mention costs them in fuel economy.

[Polchraine]

Something that I hadn't heard until relatively recently was that the U.S.A. also uses the term "Union Jack" when referring to its own flag when flown from the jackstaff of a ship.
https://en.wikipedia.org/wiki/Jack_of_the_United_States

Jacks are common throughout Navies - just con-incidence with the US being a Union too or did they copy another of our superior traditions.