The Japan Times had an interesting article about kōshitsu yōgo (皇室用語) the other day that is worth sharing. Some of the words featured in the article include:
退位 (taii,) abdication
即位 (sokui) enthronement; accede to the throne
戴冠 (taikan) coronation
皇室用語 (kōshitsu yōgo) or 最高敬語 (saikō keigo), terminology and language used exclusively for the Imperial family
陛下 (heika, lit. “at the bottom of the stairs”) is an honorific used with both 天皇 (tennō, Emperor) and 皇后 (kōgō, Empress) and is similar to saying “your majesty” in English.
皇太子殿下 (kōtaishi-denka) His Highness the Crown Prince
皇太子妃殿下 (kōtaishihi-denka) Her Highness the Crown Princess
行幸 (gyōkō) a visit by the Emperor only
行幸啓 (gyōkōkei) a visit carried out by the Imperial couple together
上皇 (jōkō) the Emperor Emeritus
上皇后 (jōkōgō) the Empress Emerita