The title Byzantine emperors commonly used after the 7th century was basileus, a Greek word meaning “king” or, in imperial usage, “emperor.”
The change reflected the Eastern Roman Empire’s growing Greek linguistic and cultural character. Earlier emperors formally relied heavily on Latin titles such as imperator, Augustus, and Caesar, even though Greek-speaking subjects commonly used basileus in speech and writing. Emperor Heraclius is associated with making basileus part of official imperial titulature around 629, after his victories over the Sasanian Persian Empire.
Basileus did not mean that Byzantine rulers abandoned their Roman identity. They continued to call themselves Roman emperors, and their subjects commonly understood the empire as the Roman Empire. Greek became increasingly dominant in administration and public life, while older titles such as autokrator, meaning “self-ruler,” continued to appear in imperial formulas.
The title also created potential ambiguity because basileus could describe other kings. Byzantine emperors were therefore often styled basileus and autokrator of the Romans. Later rulers sometimes used expanded forms such as basileus ton Rhomaion, meaning “emperor of the Romans.”