Emperor

Emperor is a title indicating an aristocratic ruler of even greater importance than a king. From the Latin imperator, originally a title for a Roman military commander.

Countries which have been ruled by emperors or empresses:

Roman empire

China

Russia ("czar" or "tsar" is from the Latin "Caesar")

Germany

[Austro-Hungarian empire]? (Holy Roman Empire

Countries that are currently ruled by an emperor or empress:

Japan