Imuhashira - Imuhashira

...This refers to a cypress pillar that is erected in the center of the floor of the shrines of both Ise Shrines. It is called the "Seiden Shinbashira" (Koutai Jingu Ritual Book), that is, it is named because it stands in the center of the shrine (Kojiki-den), but it is not a practical pillar for the shrine, and is also called the imuhashira (imuhashira) in particular, as it is a pillar that is highly valued for its purity and mysticism in the shrine's rituals. At the time of the Shikinen Sengu ceremony, which is held every 20 years, a special Kimoto-sai ceremony to cut down the wood for the pillar and a Shinomibashira ceremony to erect it are both performed as secret ceremonies at night, and the attendants are limited to certain priests such as the negi and o-monokimi.

*Some of the terminology explanations that mention "Ikibashira" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…伊勢両宮の神殿の床下中央に建てられた檜の掘立柱をいう。〈正殿心柱〉(皇太神宮儀式帳)といい，つまり社殿の中心に立つゆえの名(《古事記伝》)だが，別に社殿の実用的な支柱でなく，しかも神宮祭祀上きわめて清浄神秘を重んじられる柱として特に忌柱（いむはしら）とも称される。20年ごとの式年遷宮に当たって，特別にその用材を伐採するための木本祭と，これを建てる心御柱祭とがいずれも夜間の秘儀として執行され，奉仕者も禰宜，大物忌など特定の神職に限られる。…

※「忌柱」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報