your/his/her Royal HighnessPGOused when speaking about or to a royal person, especially a prince or princess 殿下〔对王室成员尤其是王子或公主的称呼〕
Examples from the Corpus
Royal Highness• Her Royal Highness went round the room meeting many of those who had supported this Gala evening.• Her Royal Highness who was looking radiant, assured everybody that she would be back on skis at next year's Championships.• His Royal Highness was not available for comment, except in dire necessity.