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Meaning of union jack

un'ion jack"

Pronunciation: [key]
  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. (often cap.) the British national flag.
  3. any flag the overall design of which is a union.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.