Imperial Majesty (His/Her Imperial Majesty, abbreviated as HIM) is a style used by emperors and empresses. It distinguishes the status of an emperor/empress from that of a king/queen, who are simply styled Majesty (HM). Holders of this style have sometimes been observed to follow religious leaders who are styled "His Holiness" in public ceremonies.
King-Emperors and Queen-Empresses sometimes have used the style Imperial and Royal Majesty (e.g. The Empress Frederick's style Her Imperial and Royal Majesty The German Empress and Queen of Prussia).
Incongruously, in modern times the Emperor of Japan rarely uses the style of "Imperial Majesty", instead preferring the simpler style of "Majesty". This is despite the Imperial titles still used by the rest of the Imperial household.