Refers to an emperor, especially of historical empires such as China or Rome, or in a general monarchical context. For the Emperor of Japan, 天皇 (てんのう) is the specific term.
秦の始皇帝は中国を統一した。
Qin Shi Huang, the First Emperor, unified China.
ローマ皇帝の名前をいくつか知っていますか。
Do you know the names of some Roman emperors?
He became the emperor of Rome.
He ascended to the throne as emperor at a young age.
The emperor of the Qing dynasty lived in Beijing.
After the rebellion, he declared himself emperor.