Although monarchical titles had been avoided in the Latin west, in the Greek east with, where republicanism was not as strong, the title basileus (king) was commonly applied to the Roman Emperor. In official documents basileus was more commonly applied to the Persian Shah or the rulers of the Barbarian kingdoms of the west. However, over time basileus came to mean emperor and in 629 Herakleios became the first emperor to use the title in an official capacity. This act is seen as a step in the gradual Hellenization of the empire, although Latin titles would continue to be used on seals and coins for centuries.
Basileus first appears on Byzantine seals in Latinized, plural form (basilis) in the reign of Leo III, almost a century after its adoption by Herakleios. Basileus is primarily a middle Byzantine phenomenon, and is the title used exclusively on the aniconic seals of Iconoclast emperors, typically with the adjective pistos (pious, regularly misspelled in the plural as pisty). Although the modifier Romaion, “of the Romans,” does not appear on the miliaresion until the reign of Leo V, it is present on seals from the beginning of the reign of Leo III. After deposing her son Eirene used the female form, basilissa on her seals. By the mid-tenth century, the title occasionally occurs alongside the title autokrator (possibly on a seal of Constantine VII, as well as one of Basil II). The title does not appear on seals after the reign of Nikephoros III Botaniates (1078–81).