Refers specifically to the Japanese emperor; not used for emperors of other countries.
The Emperor is the symbol of Japan.
His Majesty the Emperor's birthday is a national holiday.
The new Emperor ascended the throne.
It was unusual for the Emperor to abdicate while still alive.
The foreign ambassador had an audience with the Emperor.
天皇 refers to the emperor as a person, whereas 天皇制 refers to the institution or system surrounding the throne.
the Emperor System
the Imperial Family
emperor worship
abdicated emperor; retired emperor
female emperor (of Japan)
matrilineal emperor of Japan; emperor with imperial mother and non-imperial father
patrilineal imperial succession (Japan)
symbolic emperor; emperor as a symbol of the state
theory of the Emperor as an organ of government