The Chūgoku region (中国地方, Chūgoku-chihō), also known as the San'in-San'yō region (山陰山陽地方, San'in san'yō-chihō), is the westernmost region of Honshū, the largest island of Japan. It consists of the prefectures of Hiroshima, Okayama, Shimane, Tottori and Yamaguchi. As of 2010 it has a population of 7,563,428.
“Chūgoku” literally means “middle country”, but the origin of the name is unclear. Historically, Japan was divided into a number of provinces called koku, which were in turn classified according to both their power and their distances from the administrative center in Kansai. Under the latter classification, most provinces are divided into “near countries” (近国 kingoku), “middle countries” (中国 chūgoku), and “far countries” (遠国 ongoku). Therefore, one explanation is that Chūgoku was originally used to refer to the collection of “middle countries” to the west of the capital.
However, only five (less than half) of the provinces normally considered part of Chūgoku region were in fact classified as middle countries, and the term never applied to the many middle countries to the west of Kansai. Therefore, an alternative explanation is that Chūgoku referred to provinces between Kansai and Kyūshū, which was historically important as the link between Japan and mainland Asia.