The Union Flag, or Union Jack, is the national flag of the United Kingdom. It is so called because it combines the crosses of the three countries united under one Sovereign - the kingdoms of England and Wales, of Scotland and of Ireland (although since 1921 only Northern Ireland has been part of the United Kingdom).
The flag consists of three heraldic crosses. The cross of St George, patron saint of England since the 1270's, is a red cross on a white ground. After James I succeeded to the throne, it was combined with the cross of St Andrew in 1606. The cross saltire of St Andrew, patron saint of Scotland, is a diagonal white cross on a blue ground. The cross saltire of St Patrick, patron saint of Ireland, is a diagonal red cross on a white ground. (source: https://www.royal.uk/union-jack)