The Collaborative International Dictionary of English v.0.48:

Hokkaido \Hok*kai"do\ (h[o^]k*k[imac]"d[=o]), prop. n. [Jap.]
   The northernmost of the main islands of Japan. Together with
   the islands of Honshu, Kyushu and Shikoku it forms the bulk
   of the land area of Japan.
   [PJC]

WordNet (r) 3.0 (2006):

Hokkaido
    n 1: the second largest of the four main islands of Japan; to
         the north of Honshu [syn: Hokkaido, Ezo, Yezo]