A gongen (権現), literally "incarnation", was believed to be the manifestation of a buddha in the form of an indigenous kami, an entity who had come to guide the people to salvation, during the era of shinbutsu-shūgō in premodern Japan. The words gonge (権化) and kegen (化現) are synonyms for gongen. Gongen shinkō (権現信仰) is the term for belief in the existence of gongen.
The gongen concept is the cornerstone of the honji suijaku theory, according to which Buddhist deities choose to appear to the Japanese as native kami in order to save them, which is based on the Mahayana Buddhist notion of upaya, "expedient means".
It is sometimes assumed that the word gongen derives from Tokugawa Ieyasu's posthumous name (Tōshō Daigongen). However, the term was created and started being used in the middle of the Heian period in an effort to harmonize Buddhism and indigenous religious practice in what is called shinbutsu-shūgō or "syncretism of kami and buddhas". At that time, the assumption that Japanese kami and buddhas were essentially the same evolved into a theory called honji suijaku (本地垂迹), which held that native kami were manifestations or avatars of buddhas, bodhisattvas and other Buddhist deities. The theory gradually spread around the country and the concept of gongen, a dual entity composed of a buddha and a kami, evolved.
Under the influence of Tendai Buddhism and Shugendō, the gongen concept was adapted to religious beliefs tied to Mount Iwaki, a volcano, so that female kami Kuniyasutamahime became associated with Avalokiteśvara ekadaśamukha (Jūichimen Kannon Bosatsu, "Eleven-Faced Guanyin"), Ōkuninushi with Bhaisajyaguru (Yakushi Nyōrai) and Kuninotokotachi with Amitābha (Amida Nyōrai).
The title "gongen" started being attached to the names of kami and shrines were built within the premises of large Buddhist temples to enshrine their tutelary kami. During the Japanese Middle Ages, shrines started being called with the name gongen to underline their ties to Buddhism. For example, in Eastern Japan there are still many Mount Haku shrines where the shrine itself is called either gongen or jinja. Because it represents the application of Buddhist terminology to native kami, the use of the term was legally abolished in the Meiji Restoration with the Shinto and Buddhism Separation Order (神仏判然令, Shin-butsu Hanzenrei) and shrines began to be called jinja.Izuna Gongen (飯網の権現), also called "Izuna Myōjin" and enshrined in Izuna Shrine in Nagano, is similar to a tengu and represents the kami of Mount Iizuna.
Izusan Gongen (伊豆山権現) or Hashiri-yu Gongen (走湯権現) is the spirit of a hot spring on Izusan, a hill in Shizuoka Prefecture, enshrined in the Izusan Jinja
Kumano Gongen (熊野権現), also known as Three Mountains of Kumano (熊野三山). The kami enshrined in the three Kumano Sanzan Grand Shrines and worshipped in Kumano shrines are the three Kumano mountains: Hongū, Shingū, and Nachi.
Seiryū Gongen (清滝権現) was enshrined in Jingo-ji in Takao as the tutelary kami of Shingon Buddhism by Kūkai.
Tōshō Daigongen (東照大権現) is one of the most famous examples of gongen, representing Tokugawa Ieyasu posthumously enshrined in so-called Tōshō-gū shrines present all over Japan. The original one is Nikkō Tōshō-gū in Nikkō, Tochigi.
Zaō Gongen (蔵王権現) or Kongō Zaō Bosatsu (金剛蔵王菩薩) is a deity worshiped in Shugendō.
Gongen-zukuri (権現造) is the name of a complex Shinto shrine structure in which the haiden, or worship hall, and the honden, or main sanctuary, are interconnected under the same roof in the shape of an H. One of the oldest examples of gongen-zukuri is Kitano Tenmangū in Kyoto. The name comes from Nikkō Tōshō-gū in Nikkō because, as we have seen, it enshrines the Tōshō Daigongen and adopts this structure.