Shinboku refers to a tree or a forest as Himorogi (a temporarily erected sacred space or "altar" used as a locus of worship) in Koshinto (ancient Shinto) and an object of worship. Otherwise, shinboku refers to trees that involves roles as yorishiro (an object to spiritually rely on instead of a god), shiniki (shrine precincts or a holy area), or kekkai (the border of a spiritually limited area or prevention of invasion of spiritually evil existences).
In general, shinboku refers to a tree as an object of worship or a tree considered to be holy, in the precincts of a shrine or a high-class shrine of Jinja-Shinto; a forest surrounding a shrine to protect the shrine; or a tree that is not cut down.
In addition, shinboku refers to a tree which is in a site owned by a shrine or a private land and which has a special history such as an oral tradition. Shinboku sometimes refers to trees which are grown by forestation, have naturally grown and which are specially cut down to be used for building a shrine.
The nature worship such as himorogi and iwakura-shinko (large rock worship) is a part of the Koshinto and includes not only trees but also any symbolic object which is located at a border between environments and is treated as an object of worship because of gratitude, fear, and reverence towards gods, life, and nature. Throughout the several thousand year history of Koshinto, forms and styles for shrines as containers and for festival rites as inner substance were established independently or under the influence of foreign religions, and these were further established as jinja (shrine) shinto, etc.
Although the place where a god stays has been changed to a sanctuary of shinto called yashiro, most of several ten thousand shrines in Japan were each originally constructed at a place where there was himorogi in Koshinto and, therefore, each of those shrines has shinboku as himorogi or a spiritual stone (rock) as iwakura in its precincts and it is deified. Koshinto and jinja shinto coexist and are inseparable in some aspects and, therefore, some shrines have no buildings and deify shinboku as objects of worship, and some places are worshiped because of shinboku which have naturally grown even though there are no shrines in such places.
Kamiyorigi, kanijnboku, etc., are specially treated as yorishiro of a god and are decorated with shimenawa (a holy straw rope). Shinboku is treated as an object of worship in shiniki where there are no shrine buildings. Nagi trees, mochinoki tree and cedars are often treated as shinboku.
In addition, in some cases, trees which are considered special in addition to yorishiro in order to maintain the scenery or make an solemn atmosphere such as jingu cedars in the Ise-jingu Shrine are treated as shinboku. For those who work in mountains, conspicuous trees are temporarily treated as shinboku and deified as yorishiro of mountain gods.
Although gohei which is used in rites of jinja shinto and which is made of sakaki tree branches or nagi tree branches is also called himorogi, gohei is originally a simplified yorishiro as a substitute of naturally-grown shinboku in Koshinto.
Shiniki and kekkai
Himorogi in Koshinto considered shiniki as a place for a god to stay, or a border to the other world and this world, and was feared and respected. Shimenawa (a decorated straw rope) is put around himorogi as kekkai to prevent people or things in this world, gods in the other world, or those that invite misfortunes and mishaps for this world from easily coming over and go across, making himorogi off-limits. At present, in many places, not only shrines, shinboku, and shrine forests but entire islands are considered to be off-limits, such as Okino-shima Island. In some places, a specific time period to invite happiness is provided and rites and festivals to cancel kekkai of shinboku and to invite gods are held.
Shinboku refers to a tree which has a special episode such as an old tale. In addition, a tree such as one which is featured in a waka poem by a famous tanka poet is treated as shinboku. Many trees are treated as shinboku that are featured in waka poems by tanka poet such as the Japanese plum tree in Dazaifu-Tenmangu Shrine.
A tree which was donated by a person who is related to a shrine, etc., for some reason has been considered holy and treated as shinboku.
Trees for construction material
The trees to be used as lumber for constructing a shrine are treated as shinboku.