Definition of the Union Jack noun from the Oxford Advanced Learner's Dictionary

 

the Union Jack

 noun
noun
BrE
 
; NAmE
 
[singular]
 
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the name for the national flag of the United Kingdom Culturethe Union JackThe national flag of the United Kingdom is commonly known as the Union Jack and also as the Union Flag. It has been used as the British flag since 1603, when Scotland and England were united. The original design combined the red cross of England, St George's Cross, with the white diagonal cross on a blue background of Scotland, St Andrew's Cross. The red diagonal cross of Ireland, St Patrick's Cross, was added in 1801, when Ireland became part of the United Kingdom. Wales is not represented on the Union Jack because it is a principality of England. The national flag of Wales with the red dragon of Cadwallader on a green and white background has been recognized since the 1950s as the national flag of Wales. The national flag of Scotland, St Andrew's Cross, is also known as the Saltire.The Union Jack is most often seen flying from public buildings or at sports events. Children may wave small Union Jacks when a member of the royal family visits their town. During national celebrations strings of small flags are hung across the street as bunting.The Union Jack is less important to British people than the Stars and Stripes is to Americans. Many people feel a stronger loyalty to the national flags of England, Scotland, Wales or Northern Ireland. The flag of the European Union, a circle of gold stars on a blue background, is sometimes also seen in Britain, e.g. on car number plates.
See the Oxford Advanced American Dictionary entry: the Union Jack