The central and largest of the four main islands of Japan; between the Sea of Japan and the Pacific Ocean; regarded as the Japanese mainland WordNet 3.0 & Open English WordNet

Honshu, historically called Akitsushima, is the largest and most populous island of Japan. Wikipedia

The largest island of Japan, called the Mainland; it is south of Hokkaido across the Tsugaru Strait, north of Shikoku across the Inland Sea, and northeast of Kyushu across the Kanmon Strait. It is the seventh largest island, and the second most populous island in the world. OmegaWiki

The main island of the Japanese Archipelago. Wiktionary