Union Jack 
noun
1The national flag of the United Kingdom, formed by combining the red and white crosses of St George, St Andrew, and St Patrick and retaining the blue ground of the flag of St Andrew. Also called Union flag.No chance of buying a Union Jack, Thai, Australian, Japanese, Scottish, Welsh, American, Korean or German flag now… sold out weeks ago....
- Cheering crowds waving Union Jacks and the flag of Gurnard Sailing Club, where Shirley is a member, lined the route.
- He also has a skull and crossbones which he hoists when his grandchildren visit, a Union Jack and a Yorkshire white rose - and he has no intention of getting rid of them.
Originally a small British union flag flown as the jack of a ship
2 (union jack) (In the US) a small flag consisting of the union from the national flag, flown at the bows of vessels in harbour.