The Union Jack or Union Flag is the national flag of the United Kingdom.

The flag consists of the red cross of Saint George (the patron saint of England), edged in white, superimposed on the red saltire of Saint Patrick (the patron saint of Ireland), also edged in white, superimposed on the saltire of Saint Andrew (the patron saint of Scotland). Wales is not represented in the flag by Wales's patron saint, Saint David, because the flag was designed while Wales was part of the Kingdom of England.

The origins of the flag date to the earlier flag of Great Britain which was established in 1606 by a proclamation of King James VI and I of Scotland and England. The present design was established by an Order in Council following the Act of Union 1801, which joined the Kingdom of Great Britain and the Kingdom of Ireland to create the United Kingdom of Great Britain and Ireland. It was unchanged following the secession of the Irish Free State in 1922.

It is sometimes asserted that the term Union Jack properly refers only to naval usage, but this assertion was dismissed by the Flag Institute in 2013 after historical investigations.

The flags of British Overseas Territories, as well as certain sovereign states and regions (particularly in the Commonwealth) that were previously British possessions, incorporate the Union Jack into their own flag designs or have official flags that are derived from the Union Jack. Many of these flags are blue or red ensigns with the Union Jack in the canton and defaced with the distinguishing arms of the territory. The governors of British Overseas Territories and the Australian states also have personal standards that incorporate the Union Jack in their design. The flag continues to have official status in Canada, by parliamentary resolution, where it is also known as the Royal Union Flag.

The terms Union Jack and Union Flag are both used historically for describing the national flag of the United Kingdom.

Initially, the flag was termed the 'British flag' or 'flag of Britain', and the term 'Union' first appeared in 1625. The etymology of jack in the context of flagstaffs reaches back to Middle German. The suffix -kin was used in Middle Dutch and Middle German as a diminutive. Examples occur in both Chaucer and Langland though the form is unknown in Old English. John is a common male forename (going back to the Bible), appearing in Dutch as Jan. Both languages use it as a generic form for a man in general. The two were combined in the Middle Dutch Janke, whence Middle French Jakke and Middle English Jack. Jack came to be used to identify all manner of particularly small objects or small versions of larger ones. The Oxford English Dictionary has definition III.21 "Something insignificant, or smaller than the normal size" and gives examples from 1530 to 2014 of this usage. Further examples in the compounds section at 2b illustrate this. The original maritime flag use of jack was "A ship's flag of a smaller size than the ensign, used at sea as a signal, or as an identifying device". The jack was flown in the bows or from the head of the spritsail mast to indicate the vessel's nationality: "You are alsoe for this present service to keepe in yo^r Jack at yo^r Boultspritt end and yo^r Pendant and yo^r Ordinance" The Union Flag when instantiated as a small jack became known as the "Union Jack" and this later term transferred to more general usage of the Union Flag.

Also later a short flagpole was placed in the bows of a ship to fly the jack, this became known as the jackstaff.