Mainland Japan

Mainland Japan is the name used to describe all the regions of Japan except for Hokkaido and Okinawa. Until the end of World War II, the name included Hokkaido and Okinawa, but it excluded all the colonies of the Japanese Empire, including Korea and Taiwan. However, since the colonies are no longer under control of Japan, those places have no reason to call Japan "the mainland."
However, they are still used by residents of Okinawa and Hokkaido because their cultures and native people are different from the other parts of Japan.
In Hokkiado, the word "Dogai" is also used. It means every part of Japan outside of Hokkaido.