The Union Jack, or Union Flag, is the national flag of the United Kingdom. "Union Flag" has been the name preferred in official documents since the late 19th century. "Union Jack" is the traditional name, and remains more popular.
The flag as we know it today (although with somewhat different proportions) was introduced with the unification of Great Britain and Ireland on 1 January 1801. It is a superposition of the crosses of St. George, St. Andrew and St. Patrick, representing England, Scotland and Ireland, respectively.
The term "Union Jack" is older than this, however, being in common use by the second half of the 17th century. At this time it referred to the superposition of the English and Scottish flags which had existed in one form or another since 1606, shortly after the unification of the crowns of England and Scotland.
The Union Jack was originally a royal flag, rather than a national flag. In fact, no law has ever been passed making it a national flag, but it has become one through usage. Its first parliamentary recognition as a national flag came in 1908, when it was declared that "the Union Jack should be regarded as the National flag". A more categorical statement was made by the Home Secretary in 1933, when he stated that "the Union Flag is the National Flag".
The Union Jack is found in the canton (top left-hand corner) of the flags of some former colonies of the UK, notably Australia and New Zealand. In both countries, the Union Jack was used semi-interchangably with their national flags for significant parts of their early history.