Shinchoku (an oracle) (神勅)
Shinchoku is a word that refers to Heaven's will, and also its writing.
Tenjomukyu no Shinchoku (the oracle that is as eternal as heaven and earth)
In general, Shinchoku refers to following three shinchoku (Three Great Shinchoku) that were given by Amaterasu Omikami (the Sun Goddess) to her grandson Ninigi (God of the Harvest) in the passage of Tensonkorin (the descent of the Sun-Goddess's grandson) recorded in "Nihonshoki" (Chronicles of Japan).
Tenjomukyu no Shinchoku
Ashihara Chihoaki Mizuho no Kuni (the eulogistic name for Japan; the Land of Abundant Reed Plains and Rice Fields), this is the land where my descentant shall reign. Imashi Sumemina (dear my child who carries out the will of the God of Heaven), take and fulfill the mission given, and be the one to reign. May you be filled with happiness. The grace and goodness of the God of Heaven shall prosper and last eternally with heaven and earth.
Hokyohosai no Shinchoku (the oracle of hokyo (sacred mirror) dedication)
My child, treat the hokyo as you have treated me. Enshrine the mirror at your side as a goshintai (object of worship housed in a Shinto shrine and believed to contain the spirit of a deity) of Amaterasu Omikami.
Yuniwa no Inaho no Shinchoku (the oracle of the rice ear from Yuniwa (sacred field of the God))
The rice ears grown by Amaterasu Omikami in the Heaven shall be handed to children, so grow them eternally in Mizuho no Kuni (the Land of Vigorous Rice Plants; Japan).
Including 'Jidenbogo no Shinchoku' (commanding; the people shall never neglect loyalty towards Imperial family) and 'Himorogi Iwasaka no Shinchoku' (commanding; saishi (religious service) shall be based on prosperity of Imperial line) that Amaterasu Omikami commanded to her subject Amenokoyane no Mikoto and Futodama no Mikoto in the same passage, it is called 'Godai Shinchoku' (Five Great Shinchoku).
The passage of Tensonkorin (the descent of the Sun-Goddess's grandson) recorded in "Kojiki" (The Records of Ancient Matters) also include a similar sentence regarding Tenjomukyu no Shinchoku. Amaterasu Omikami entrusted and told, this Toyo Ashihara Mizuho no Kuni (the Land of Abundant Reed Plains and Rice Fields) is the land you reign'. Although the sentences are different from each other, it shares the common context that it is the will of God for Ninigi-no Mikoto (grandson of Amaterasu Omikami) and his descendants to become the monarch and reign over Japan. It used to be considered that the position was succeeded to Imperial family since Iwarehikono, the great-grandson of Ninigi-no Mikoto, accede to the throne as Emperor Jimmu. In the prewar period, it was considered as a legal, historical and religious evidence (See 'Unbroken Imperial Line' for the reference) of the Emperor being the monarch of Japan.
Shinchoku was a concept that received less emphasis, except for nationalistic scholars of the Japanese classics, before modern times. However, after the end of the Meiji period, with Japan's modernization advancing rapidly, the movements aiming to introduce a communist system of government and those aiming to introduce a republican one expanded, resulting in the theory that the Emperor was an organ of the state becoming the mainstream of Constitutional Theory; as a result, Shinchoku was often used as a rationale for opposing these movements. Especially during the war period, it was related to Kokoku Shikan (Emperor-centered historiography which is based on state Shinto) and national policy theory, to be used as one of foundation to support government authorized idea.