The Union Flag, also known as the Union Jack, is the flag of the United Kingdom. It retains an official or semi-official status in some Commonwealth Realms; for example, it is known as the Royal Union Flag in Canada. It is also used as an official flag in some of the smaller British overseas territories. The current design dates from the Union of Great Britain and Ireland in 1801.

Whether to call it the "Union Flag" or the "Union Jack" is a matter of debate by many. According to the Flag Institute, the vexillological organisation for the United Kingdom, "the national flag of the United Kingdom, the Crown Dependencies and Overseas Territories is the Union Flag, which may also be called the Union Jack." It also notes that "From early in its life the Admiralty itself frequently referred to the flag as the Union Jack, whatever its use, and in 1902 an Admiralty Circular announced that Their Lordships had decided that either name could be used officially. In 1908, a government minister stated, in response to a Parliamentary question, that "the Union Jack should be regarded as the National flag" . Nevertheless, the term "Union Flag" is used in King Charles's proclamation of 1634, and in King George III's proclamation of 1 January 1801 concerning the arms and flag of the United Kingdom of Great Britain and Ireland.