Collins Concise English Dictionary © HarperCollins Publishers::
Union Jack n
  1. a common name for Union flag
WordReference Random House Unabridged Dictionary of American English © 2020
union jack′, 
  1. a jack consisting of the union of a national flag or ensign, as the U.S. jack, which has the white stars and blue field of the union of the U.S. national flag.
  2. British Terms(often cap.) the British national flag.
  3. any flag the overall design of which is a union.
  • 1665–75

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