In Shinto, shintai (神体 body of the kami), or go-shintai (御神体 sacred body of the kami) when the honorific prefix go- is used, are physical objects worshipped at or near Shinto shrines as repositories in which spirits or kami reside. Shintai used in Shrine Shinto (Jinja Shinto) can be additionally called mitamashiro (御霊代 spirit replacement or substitute).
In spite of what their name might suggest, shintai aren't themselves part of kami, but rather just temporary repositories which make them accessible to human beings for worship. Shintai are additionally of necessity yorishiro, that's objects by their quite nature capable of attracting kami.
The most common shintai are man-made objects like mirrors, swords, jewels (for example comma-shaped stones called magatama), gohei (wands used throughout religious rites), and sculptures of kami called shinzō (神像), but they can be additionally natural objects such as stones, mountains, trees and waterfalls. Before the forcible separation of kami and Buddhas of 1868 (shinbutsu bunri) a shintai could even be the statue of a Buddhist deity.
Famous shintai include the mirror (part of the Imperial Regalia of Japan), Mount Miwa, Mount Nantai, the Nachi Falls, and the Meoto Iwa rocks. Many mountains like Mount Miwa or the Three Mountains of Kumano (Kumano sanzan) are considered shintai and are therefore called shintaizan (神体山, shintai mountain). The most famous shintai of all is doubtless Mount Fuji.
A yokozuna, a wrestler at the top of sumō's power pyramid, is a living shintai. For this reason, his waist is circled by a shimenawa, a sacred rope which protects sacred objects from evil spirits. A kannushi, that is, a Shinto priest, can become a living shintai when a kami enters his body throughout religious ceremonies.
The founding of a new shrine requires the presence of either a pre-existing, naturally occurring shintai (for example a rock or waterfall housing a local kami), or of an artificial one, which must therefore be procured or made to the purpose. An example of the first case are the Nachi Falls, worshipped at Hiryū Shrine near Kumano Nachi Taisha and believed to be inhabited by a kami called Hiryū Gongen. In the second, the mitama (spirit) of a kami is divided in half through a process called kanjō and one of the halves is then stored in a yorishiro. This is the process which has led to the creation of networks of shrines housing the same kami, as for example the Hachiman shrine, Inari shrine or Kumano shrine networks.
Because over the years the shintai is wrapped in more and more layers of precious cloth and stored in more and more boxes without being ever inspected, its exact identity might become forgotten.
The first role of a shrine is to house and protect its shintai and the kami which inhabits it. If a shrine has more than one building, the one containing the shintai is called honden; because it is meant for the exclusive use of the kami, it is always closed to the public and isn't used for prayer or religious ceremonies. The shintai leaves the honden only throughout festivals (matsuri), when it is put in a "divine palanquin" (mikoshi, a term usually translated in English as "portable shrine"), and carried around the streets among the faithful. The portable shrine is used to physically protect the shintai and to hide it from sight.