The Collaborative International Dictionary of English v.0.48:

Honshu \Hon"shu\ prop. n. [Jap.]
   the main island of Japan. Together with the islands of
   Hokkaido, Kyushu and Shikoku it forms the bulk of the land
   area of Japan.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

Honshu
    n 1: 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 [syn: Honshu, Hondo]