The Great War is a battle honour awarded for service during World War I. It was awarded to units of the Canadian Army which had arrived in the United Kingdom but had not deployed to France. The honour is displayed on that unit's colours, with the dates of service.
According to General Orders of the Canadian Army, the criterion for award of the honour was as follows:
Upon the Colours of units which proceeded to England but did not reach France will be emblazoned "THE GREAT WAR", followed by year-dates. The year-dates will be determined by the calendar years, up to 1918, inclusive, during any portion of which the unit served as a unit out of Canada. (G.O. 6 / 1928)
References
The Great War (battle honour) Wikipedia(Text) CC BY-SA