were officials in Classical Japan sent from the central government to oversee a province from around the 8th century, after the enactment of the Ritsuryō

Ritsuryo

is the historical law system based on the philosophies of Confucianism and Chinese Legalism in Japan. The political system in accord to Ritsuryō is called "Ritsuryō-sei"...

 system. Kokushi held considerable power and responsibility according to the Ritsuryō

Ritsuryo

is the historical law system based on the philosophies of Confucianism and Chinese Legalism in Japan. The political system in accord to Ritsuryō is called "Ritsuryō-sei"...

, including tax collection, etc.
The highest level for a Kokushi was Kami (守). So, for instance, the kokushi of Kai would have the title of Kai no Kami (甲斐守). In some cases, the Kami himself was living directly in the province he was charged with, delegating his powers to lower ranking officials. The highest official effectively in charge of the province was called Zuryō (受領).

Kokushi lost their power during the Kamakura Shogunate

Kamakura shogunate

The Kamakura shogunate was a military dictatorship in Japan headed by the shoguns from 1185 to 1333. It was based in Kamakura. The Kamakura period draws its name from the capital of the shogunate...

 and furthermore during the Muromachi Shogunate

Muromachi period

The is a division of Japanese history running from approximately 1336 to 1573. The period marks the governance of the Muromachi or Ashikaga shogunate, which was officially established in 1338 by the first Muromachi shogun, Ashikaga Takauji, two years after the brief Kemmu restoration of imperial...

 to the Shugo

Shugo

was a title, commonly translated as "Governor," given to certain officials in feudal Japan. They were each appointed by the shogun to oversee one or more of the provinces of Japan...

. In subsequent generations, especially in the Edo period, a kokushi title remained as an honorific title.