What Latin title meant 'emperor' in Charlemagne's imperial style?

The Latin title meaning “emperor” in Charlemagne’s imperial style was imperator. In full formulations, Charlemagne could be described with titles such as imperator Romanorum, “emperor of the Romans.”

The word began in Roman usage as a military acclamation for a victorious commander. Over time, especially under the Roman Empire, imperator became associated with the emperor’s governing authority. Charlemagne adopted the term after Pope Leo III crowned him in Rome on Christmas Day 800.

Charlemagne’s own official style was more elaborate than the single word. His documents combined imperial language with his existing titles as king of the Franks and Lombards. Coins issued for him used forms such as Karolus Imperator Augustus, linking his rule to Roman imperial tradition.

The alternatives are Latin titles of different ranks or offices: rex means king, dux means duke or military leader, and comes means count or companion. They are not synonyms for imperator.