Wikipedia
Gongen
A , 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 and are synonyms for gongen. 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".