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Under Roman rule, the term basileus came to be used, in the Hellenistic tradition, to designate the Roman Emperor in the everyday and literary speech of the Greek-speaking Eastern Mediterranean.

Although the early Roman Emperors were careful to retain the facade of the republican institutions and to not formally adopt monarchical titles, the use of basileus amply illustrates that contemporaries clearly perceived that the Roman Empire was a monarchy in all but name.

Nevertheless, despite its widespread use, due to its " royal " associations the title basileus remained unofficial for the Emperor, and was restricted in official documents to client kings in the East.

Instead, in official context the imperial titles Caesar Augustus, translated into Greek as Kaisar Sebastos or Kaisar Augoustos, and Imperator, translated as Autokratōr, were used.