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.

  [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.