emperor
皇 centers on the supreme ruler: the emperor. It conveys the idea of sovereign authority and imperial status.
皇 combines 白 (white) and 王 (king), suggesting a shining or illustrious ruler. The exact historical development is uncertain, but the component combination is well established.
The top 白 (white) shines above the bottom 王 (king), like a radiant emperor whose authority is pure and supreme.
For コウ, imagine an emperor wearing a golden crown: crown -> コウ, and the emperor's head is adorned with it.
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Emperor of Japan
emperor
divine; imperial
Imperial household
crown prince
imperial family; royalty
Pope
His Majesty the Emperor
empress (consort)
imperial prince
Imperial Palace (of Japan); imperial residence
imperial princess
retired emperor; ex-emperor; former emperor; emperor emeritus
Emperor Meiji (1852-1912, reigned 1867-1912); Emperor Mutsuhito
Empire (of Japan); the (Japanese) Empire; Japan
imperial throne
Emperor's trophy
cloistered emperor; ex-emperor who has become a monk
Emperor's Birthday (national holiday; February 23)
tomb of Japanese emperor; Japanese imperial burial site