The "Union Jack" is the "FLAG" of the United Kingdom so to speak.

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cairina.moschata | Rank: Albert Einstein (7,945)
11 minutes after the question was opened (05/11/2006 11:51pm)
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I believe it is the royal ensign on borad a ship. It used to be that it was the union Jack on a ship because the part that held the flag was the jack staff but in 1908 it was accepted as the name of the national flag

helpfulTallScotsGuy | Rank: Bachelor (152)
9 hours after the question was opened (05/12/2006 08:33am)
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Yes. It was after the Union of the Crowns that the flags of England and Scotland were "looked at". The navy wanted to know which flag to fly and the answer was "both of them"...so some bright spark made one up incorporating the two of them together and said "right lads,take the flag of the new union and run it up the jack" (as mentioned the "Jack staff" was the ships flagpole)
As sailors were known then as "Jolly Jack tars" the name of the flag as the Union Jack stuck.