The Western Front was applied to both World Wars. In both cases, Germany became engaged in a two-front war, one front in the East, and one in the West.
In World War I, the Eastern Front was against Czarist Russia, and the Western Front was in France.
In World War II, the Eastern Front was against the Soviet Union (Communist Russia), and the Western Front was against the US, Canada, Britain and France, and shifted its position across Europe over the course of the war.
Often, references to the Western Front refer to the Western Front of World War I, owing in part to the Erich Maria Remarque's 1928 novel about World War I called All's Quiet on the Western Front.