A torii is a traditional Japanese gate commonly found at the entry to a Shinto shrine, although it can be found at Buddhist temples as well, such as at Hase-dera in Kamakura.
The basic structure of a torii is two columns that are topped with a horizontal rail called the kasagi. Slightly below the top rail is a second horizontal rail called the nuki. Torii are traditionally made from wood and are frequently painted vermilion. When unbarked logs are used for the torii it is called a kuroki, or "black wood" torii. Today, torii made of stone, metal or stainless steel can be found as well.
Torii mark the transition from the sacred (the shrine) to the profane (the normal world). Inari shrines typically have many torii. A person who has been successful in business often donates a torii in gratitude. The Fushimi Inari shrine in Kyoto has thousands of such torii.
The origin of torii is unclear, but there are several different theories. They may have originated in India as a derivative of the torana gates in the monastery of Sanchi, which is located in central India. In this theory, the torana was adopted by Shingon Buddhism founder Kukai, who used it to demarcate the sacred space used for the homa ceremony.
Other scholars believe that they are related to the bairou(牌楼) in China or the hongsalmun(紅箭門) in Korea.
The origin of the word "torii" is also unknown. One theory is that it was designed as a large bird perch, as hinted by the kanji, which may be derived from 鶏居 meaning 'bird perch'. This is because in Shinto, birds are considered messengers of the gods. A second theory is that it is derived from the term toori-iru (通り入る: pass through and enter).