Saijin (祭神)

Saijin is a Shinto term that signifies a deity enshrined in a Shinto shrine.

History
Today, many shrines have gods from Japanese mythology as their Saijin or share the same gods as those of Japanese mythology.

With its origins in the worship of nature, Shinto holds the seas, mountains, and rivers sacred as shintai (an object of worship at a Shinto shrine that is believed to represent the spirit of a deity) and in early Shinto shrines, either no particular names were given to such enshrined deities or their names are unknown. Only a few shrines, such as Ise-jingu Shrine or Sumiyoshi-jinja Shrine, had Saijin whose names were expressly recorded in the Kojiki (the Records of Ancient Matters), the Nihonshoki (Chronicles of Japan), or the Manyoshu (Collection of Ten Thousand Leaves); most Saijin were just called by the name of the place of enshrinement or by the name of the shrine with the word "kami "(god) added as a suffix. The Jinmyocho (list of deities) in the Engishiki (codes and procedures on national rites and prayers) list most shrines by name only, showing that Saijin of most shrines were not specifically named until around the early 10th century, when the Engishiki was compiled.

From around the 10th century, god with specific virtues and attributes such as the god of fire, the god of water and the god of trees, were added to the existing figures of Ujigami (a guardian god or spirit of a particular place) and Jinushigami (god of an area of land). Likewise, Saijin became anthropomorphic deities with names derived from gods and goddesses of Japanese mythology or with names ending with 'Mikoto,' 'hiko' or 'hime' instead of having just place names or shrine names. Also, it became common that Shinto shrines prayed for the coming of powerful gods from Inari-jinja Shrine or Hachiman-gu Shrine to be their Shushin (shushin).

Shushin, Haishin
Generally more than one Saijin are enshrined in a shrine. Among them, the chief god, that is to say, the main enshrined deity, is called "Shushin"or "Shusaijin," with other deities referred to as "Haishin,""Haishishin"or "Aidonoshin."

The distinction between Shushin and Haishin originated in the Meiji period with the introduction of a system of ranks for official shrines, although the concept of 'a chief god and other gods' already existed, with the main deity known as 'Mae' and others as 'Aidonoshin.'
Haishin usually have some connection to the Shushin but sometimes they were enshrined together for obscure reasons. Some Haishin were enshrined concurrently with Shushin, and some were added later. This type includes deities who were originally Haishin and later became Shushin. When shrines were forced to merge in the Meiji period, some shrines came to enshrine many Haishin.

Aidono (相殿, also written as 合殿) is a building in a Shinto shrine which enshrines several deities, including the Shushin.
Deities enshrined in the Aidono are referred to as 'Aidonoshin.'
If the shrine enshrines both Shushin and Haishin, Haishin are called Aidonoshin.