The national banner of Great Britain and Ireland. It consists of three united crosses- that of St. George for England, the saltire of St. Andrew for Scotland, and the cross of St. Patrick for Ireland.
In the Union Jack the white edging of St. George's cross shows the white field. In the saltire the cross is reversed on each side, showing that the other half of the cross is covered over. The broad white band is the St. Andrew's cross; the narrow white edge is the white field of St. Patrick's cross.
In regard to the word “Jack,” some say it is Jacque (James), the name of the king who united the flags, but this is not correct. Jacque is a surcoat emblazoned with St. George's cross. James I. added St. Andrew's cross, and St. Patrick's cross was added in 1801. (Jaque, our “jacket.”)
Technically described thus:
“The Union Flag shall be azure, the Crosses saltire of St. Andrew and St. Patrick quarterly per saltire counter changed, argent and gules, the latter fimbriated of the second, surmounted by the Cross of St. George of the third, fimbriated as the saltire.” —By order of the Council.
“Jaque, de l'allemand jacke, espece de petite casaque
militaire qu'on portait au moyen fâge surles armes et sur la
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894