The word 'hoshu' (also referred as 'hossu') refers to a person who keeps the creed of Buddhism and plays a central role in its teaching, and turns to refer to the supreme leader (head) of the school, sect and religious community.
It originally referred to Shakyamuni. As in the 49th Daiku-kyo Sutra (大空経) of the Chu Agon-kyo Sutra (The Middle-Length Agama Sutras), there is a description that 'make Shakyamuni the base of Buddhism and hoshu. Buddhism should be preached by Shakyamuni,' and in the first of the Zo Agon-kyo (Miscellaneous Agama Sutra) there is a description that 'make Shakyamuni hoshu, a guide and fuku (覆),' it was a honorific title of Shakyamuni, a master of Shobo (the True Dharma of Buddhism).
In China a high priest who taught Buddhism with high virtue was called hoshu, which was introduced to Japan. In the "Jinno Shotoki" (A Chronicle of Gods and Sovereigns), the Emperor Uda (after abdication of the throne, became a priest and founded the Ninna-ji Temple) who lived in the Heian period was called 'hoshu of the two lineages' (secular affairs and Buddhism). After medieval times, the supreme leaders such as chief priests of the sect and religious community which had been succeeded for generations by kechimyaku sojo (Transmission of the Heritage; the secret transmission from the Great Master to only one disciple) and the head priests of the grand head temple had been called hoshu as a honorific title.