The Union Flag, commonly known as the Union Jack, is the flag of the United Kingdom, as well as a flag with an official or semi-official status in some Commonwealth realms. It is also used as an official flag in some of the smaller British overseas territories. The Union Flag also appears in the canton (upper left-hand quarter) of the flags of several nations and territories that were former British colonies.
The current design dates from the Union of Great Britain and Ireland in 1801. The flag combines aspects of three national flags: the red cross of Saint George, the red saltire of Saint Patrick's Flag, and the Flag of Scotland.
Thank you, Irmina!