Kokutai is a concept in the Japanese language translatable as "system of government", "sovereignty", "national identity, essence and character", "national polity; body politic; national entity; basis for the Emperor's sovereignty; Japanese constitution" or nation.

The body politic is a polity—such as a city, realm, or state—considered metaphorically as a physical body. Historically, the sovereign is typically portrayed as the body's head, and the analogy may also be extended to other anatomical parts, as in political readings of Aesop's fable of "The Belly and the Members". The image originates in ancient Greek philosophy, beginning in the 6th century BC, and was later extended in Roman philosophy. Following the high and late medieval revival of the Byzantine Corpus Juris Civilis in Latin Europe, the "body politic" took on a jurisprudential significance by being identified with the legal theory of the corporation, gaining salience in political thought from the 13th century on. In English law the image of the body politic developed into the theory of the king's two bodies and the Crown as corporation sole.