"Mainland Japan" is a term used to distinguish Japan's core land area from its outlying territories. It is most commonly used to distinguish the country's four largest islands from smaller islands such as the Ryukyu Islands and Ogasawara Islands, although depending on the context the term "Mainland Japan" might refer only to Honshū, the largest island.

The Empire of Japan, also referred to as the Japanese Empire, Imperial Japan, or simply Japan, was the Japanese nation-state that existed from the Meiji Restoration in 1868 until the enactment of the reformed Constitution of Japan in 1947. From 29 August 1910 to 2 September 1945, the Empire of Japan included the naichi and the gaichi. It also ruled colonies such as Kwantung, South Seas, Mantetsu, and its concessions. The naichi was under Japanese direct rule while the gaichi was not, however both were parts of the empire. In the closing stages of World War II, with Japan defeated alongside the rest of the Axis, on 2 September 1945 the formalized Japanese Instrument of Surrender was issued in compliance with the Potsdam Declaration of the victorious Allies; and Japanese territory was immediately much reduced when lost Taiwan, Korea, Kuril and Karafuto as it is today.