“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.

Historically, Chūgoku referred to the 16 provinces of San’indō (山陰道) and San’yōdō (山陽道), which led to the region’s alternative name described below. However, because some of the easternmost provinces were later subsumed into prefectures based primarily in Kansai, those areas are, strictly speaking, not part of the Chūgoku region in modern usage.

In Japanese, the characters 中国 and the reading Chūgoku began to be used to mean “China” after the founding of the Republic of China. The same characters are used in Chinese to refer to China, but pronounced Zhōngguó, lit. “Middle Kingdom” or “Middle Country” (Wade Giles: Chungkuo).

Primarily in the tourism industry, in order to avoid confusing the Chūgoku region with China, the Chūgoku region is also called the “San’in‐San’yō region”. San’in (“yīn of the mountains”) is the northern part facing the Sea of Japan. San’yō (“yáng of the mountains”) is the southern part facing the Seto Inland Sea. These names were created using the yīnyáng‐based place‐naming scheme.

The Chūgoku region consists of the following prefectures: Hiroshima, Yamaguchi, Shimane, and Tottori. Okayama is also included, although only Bitchū Province was considered a Middle Country; Mimasaka Province and Bizen Province, the other two components of modern-day Okayama, were considered Near Countries. Kyūshū, Shikoku, and Kansai neighbor the Chūgoku region.

The Chūgoku region is characterized by irregular rolling hills and limited plain areas and is divided into two distinct parts by mountains running east and west through its center.