The district (郡, gun^?) was most recently used as an administrative unit in Japan between 1878 and 1921 and is roughly equivalent to the county of the United States, ranking at the level below prefecture and above city, town or village. As of 2008, cities belong directly to prefectures and are independent from districts. In Japan towns and villages belong to districts and the districts possess little to no administrative authority. The districts are used primarily in the Japanese addressing system and to identify the relevant geographical areas and collections of nearby towns and villages.

The district was initially called kōri and has ancient roots in Japan. Although the Nihon Shoki says they were established during the Taika Reforms, kōri was originally written 評.^{[citation needed]} It was not until the Taihō Code that kōri came to be written 郡. Under the Taihō Code, the administrative unit of province (国; kuni) was above district, and the village (里; sato or 郷; sato) was below.

Because district names had been unique within a single province and as of 2008 prefecture boundaries are roughly aligned to provincial boundaries, most district names are unique within their prefectures.

Japan ⁱ/dʒəˈpæn/ (Japanese: 日本 Nihon or Nippon; formally 日本国  Nippon-koku or Nihon-koku, literally the State of Japan) is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south. The characters that make up Japan's name mean "sun-origin", which is why Japan is sometimes referred to as the "Land of the Rising Sun".

Japan is an archipelago of 6,852 islands. The four largest islands are Honshū, Hokkaidō, Kyūshū and Shikoku, together accounting for ninety-seven percent of Japan's land area. Japan has the world's tenth-largest population, with over 127 million people. The Greater Tokyo Area, which includes the de facto capital city of Tokyo and several surrounding prefectures, is the largest metropolitan area in the world, with over 30 million residents.

Archaeological research indicates that people lived in Japan as early as the Upper Paleolithic period. The first written mention of Japan is in Chinese history texts from the 1st century AD. Influence from other nations followed by long periods of isolation has characterized Japan's history. In the late 19th and early 20th centuries, victory in the First Sino-Japanese War, the Russo-Japanese War and World War I allowed Japan to expand its empire during a period of increasing militarism. The Second Sino-Japanese War of 1937 expanded into part of World War II in 1941, which came to an end in 1945 following the atomic bombings of Hiroshima and Nagasaki. Since adopting its revised constitution in 1947, Japan has maintained a unitary constitutional monarchy with an emperor and an elected parliament called the Diet.