which word means heavenly prince in japanese

The most likely Japanese word is 天王 (tennō), which can be translated as “heavenly prince” or “heavenly king” in some contexts.

For a Japanese mythological title, 天孫 (tenson) is also related to “heavenly descendant,” but it does not literally mean “heavenly prince”.

If you meant the specific deity from Japanese mythology, the phrase often used is 天之忍穂耳命 or references to 邇邇芸命 (Ninigi-no-Mikoto), who is sometimes described in English as a heavenly prince, rather than that exact phrase being a standard Japanese word.