A torii is a traditional Japanese gate most commonly found at the entrance of or within a Shinto shrine, where it symbolically marks the transition from the profane to the sacred.
The presence of a torii at the entrance is usually the simplest way to identify Shinto shrines, and a small torii icon represents them on Japanese road maps. They are however a common sight at Japanese Buddhist temples too, where they stand at the entrance of the temple's own shrine, called chinjusha and are usually very small.
The function of a torii is to mark the entrance to a sacred space. For this reason, the road leading to a Shinto shrine (sandō) is almost always straddled by one or more torii, which are therefore the easiest way to distinguish a shrine from a Buddhist temple. If the sandō passes under multiple torii, the outer of them is called ichi no torii. The following ones, closer to the shrine, are usually called, in order, ni no torii and san no torii. Other torii can be found further into the shrine to represent increasing levels of holiness as one nears the inner sanctuary (honden), core of the shrine. Also, because of the strong relationship between Shinto shrines and the Japanese Imperial family, a torii stands also in front of the tombs of each Emperor.
The torii is also sometimes used as a symbol of Japan in non-religious contexts. For example, it is the symbol of the Marine Corps Security Force Regiment and the 187th Infantry Regiment, 101st Airborne Division and of other US forces in Japan.