Emperor
The English word comes from Latin, the language of the old Roman Empire. At first, an imperator was a powerful general (army leader) but, after Augustus, it was only used by their most powerful rulers. There have been many countries in history whose leaders are called "emperors" in English. The leaders of the Byzantine Empire (basileus, Autokrator) in Greece, the Holy Roman Empire (imperator, Kaiser) in Austria and Germany, Russian Empire, Serbia , and Bulgaria (tsar), and the Ottoman Empire (sultan, padishah, khagan, kaysar) in Turkey all said they were just new parts of the old Roman Empire.