Your Highness; His Highness; Her Highness
Honorific title used for imperial princes and princesses, and for certain other high-ranking royalty. Can be used as a form of address or as a reference. The reading てんが is obsolete.
His Imperial Highness the Crown Prince attended the ceremony.
Your Highness, may we have a few words?
His Imperial Highness the Crown Prince has arrived.
Your Highness, how are you feeling?
殿下 is a modern honorific used for princes and princesses, whereas 尊（みこと） is an archaic honorific suffix found in myths and old chronicles.
陛下 is reserved for the reigning Emperor or Empress, while 殿下 is used for princes, princesses, and other non-reigning royalty.
閣下 addresses high-ranking officials or ambassadors, whereas 殿下 addresses members of the royal family such as princes and princesses.
princess; Her Royal Highness
Standard kanji spelling; used for princes and princesses of the imperial family and for certain other high-ranking royalty.
From Chinese 殿下 (diànxià), literally 'below the palace', used as a respectful indirect reference to royalty.