N-VOC
殿下（对除王、后以外王室成员的尊称）
Expressions such as 'Your Highness' or 'His Highness' are used to address or refer to a member of the royal family other than a king or queen.
例句
That would be best, Your Highness...
那样最好了，殿下。
Her Royal Highness the Duchess of Kent opened an art exhibition recently.
肯特公爵夫人殿下最近为一个艺术展览揭幕。