noun, noun, used as a suffix
Your Majesty; His Majesty; Her Majesty
Honorific title used for the current emperor or a reigning monarch. Can be used as a standalone noun or as a suffix attached to the monarch's name or title.
陛下は式典にご出席になった。
His Majesty attended the ceremony.
天皇陛下にお会いできるとは光栄です。
It is an honor to meet His Majesty the Emperor.
殿下 is used for princes, princesses, and other non-reigning imperial or royal family members, while 陛下 is reserved for the reigning monarch.
From Chinese 陛下 (bìxià), originally meaning 'below the steps' (of the throne), used as an indirect respectful address for the emperor.