Final answer:
The Allied invasion of Normandy was considered the "second front" because it diverted German forces from the eastern front and alleviated pressure on the Soviet Union.
Step-by-step explanation:
The Allied invasion of Normandy, also known as D-Day, was considered the "second front" because it created a new front in the European theater of World War II. Prior to the invasion, the main front was in the east where the Soviet Union was heavily engaged in fighting the German forces. The invasion of Normandy diverted German forces from the eastern front and relieved some of the pressure on the Soviet troops, allowing them to continue their advance towards Germany.