What was the Allied plan for a full-scale invasion of Japan in World War II called?

The Allied plan for a full-scale invasion of Japan was called Operation Downfall.

Operation Downfall was the proposed invasion of the Japanese home islands if Japan did not surrender. It had two principal stages: Operation Olympic, aimed at southern Kyūshū, and Operation Coronet, aimed at the Kantō Plain near Tokyo. Olympic was planned for November 1945, with Coronet scheduled for March 1946.

The invasion never took place. Japan accepted the Allied surrender terms in August 1945, after the atomic bombings of Hiroshima and Nagasaki and the Soviet Union’s declaration of war. The cancellation of Downfall avoided the enormous casualties that planners expected from an invasion of Japan.

Operation Overlord, by contrast, was the Allied invasion of Normandy in June 1944. Olympic and Coronet were components of Downfall, not alternative names for the entire plan.