Etymology

Before World War II, the war was also known as The Great War, The World War, or The War in Europe. In France and Belgium, it was sometimes referred to as La Guerre du Droit (the War for Justice) or La Guerre Pour la Civilisation / de Oorlog tot de Beschaving (the War to Preserve Civilisation), especially on medals and commemorative monuments. The term of choice used by official histories of the war in Britain and Canada is First World War, while American histories generally use the term World War I.
The earliest known use of the term First World War appeared in September 1914 when German biologist and philosopher Ernst Haeckel said, "There is no doubt that the course and character of the feared "European War" ... will become the first world war in the full sense of the word."
The terms World War I and First World War both became standard (in the United States and Britain respectively) beginning in about 1940 to 1942; prior to that, it was most commonly called The Great War.