(Greek, Ancient (to 1453);: σεβαστός|sebastós|venerable one, Augustus, pronounced as /grc-x-byzant/; plural Greek, Ancient (to 1453);: σεβαστοί, in Greek, Modern (1453-); pronounced as /sevasˈty/) was an honorific used by the ancient Greeks to render the Roman imperial title of Latin: [[Augustus (honorific)|Augustus]]. The female form of the title was (Greek, Ancient (to 1453);: σεβαστή). It was revived as an honorific in the 11th-century Byzantine Empire, and came to form the basis of a new system of court titles. From the Komnenian period onwards, the Byzantine hierarchy included the title sebastos and variants derived from it, like,,, and .
The term appears in the Hellenistic East as an honorific for the Roman emperors from the 1st century onwards, being a translation of the Latin Latin: [[Augustus (honorific)|Augustus]]. For example, the Temple of the Sebastoi in Ephesus is dedicated to the Flavian dynasty. This association also was carried over to the naming of cities in honor of the Roman emperors, such as Sebaste, Sebasteia and Sebastopolis.
The epithet was revived in the mid-11th century—in the feminine form —by Emperor Constantine IX Monomachos for his mistress Maria Skleraina, to whom he accorded quasi-imperial honours. A number of individuals were qualified as thereafter, such as Constantine Keroularios, or Isaac Komnenos and his brother, the future emperor Alexios I Komnenos .
When the latter assumed the Byzantine throne in 1081, he set about to reorganize the old system of court dignities, with as the basis for a new set of titles—, and,, and —which primarily signalled the closeness of their holders' familial relationship to the emperor, either by blood or by marriage. This process profoundly transformed the very nature of Byzantine aristocracy, with the imposition of an entire class of imperial relatives and associates superimposed on the "traditional" administrative system and the higher officialdom that constituted the Senate. In the words of historian Paul Magdalino, this move further isolated the imperial family from the common people and made them "partners in, rather than executives of, imperial authority". In this context, the scholar L. Stiernon calculated that in the period from the late 11th to the end of the 12th century, 30% of all belonged to the ruling Komnenos family, 20% to the closely allied Doukas clan, and another 40% to other families of the high aristocracy who intermarried with the Komnenoi, the remaining 10% encompassing both Byzantines as well as foreigners who either intermarried with the imperial family or received the title as an honorific distinction.
Initially the formed the basis of this new familial aristocracy, with sons of a, a, or a being themselves; due to the proliferation of the title, however, under Manuel I Komnenos a new class of dignitaries was created for the emperor's nephews and cousins, i.e., the sons of higher dignitaries; and the were relegated to a grade below them, above the . The were further divided in two groups: the simple and the . The latter were members of various aristocratic families tied to the emperor via marriage to his female relatives (means 'son-in-law in Greek). The thus formed the upper layer of the class, but should not be confused with the imperial, the actual sons-in-law of the emperor, who were even higher in the hierarchy, ranking above the cousins and nephews and just below the . The forms ('venerable by all'), and are also found in seals, inscriptions, and correspondence of the period, but they are merely rhetorical augmentations of the original title, and do not, as was believed by earlier scholars like Gustave Schlumberger, represent distinct and superior ranks. It is notable that among Byzantine, their precedence was not determined by the offices they might bear, but by the degree of their kinship to the emperor.
The title was also conferred to foreign rulers, and spread to neighboring, Byzantine-influenced states, like Bulgaria, where a was the head of an administrative district, and Serbia, where the title was employed for various officials.
In Byzantium itself, the title lost its pre-eminence in the late 12th century, and in the following centuries the was a title reserved for commanders of ethnic units. By the time pseudo-Kodinos wrote his Book of Offices, shortly after the middle of the 14th century, the sebastos occupied one of the lowest rungs in the imperial hierarchy, coming 78th between the and the . His court dress was a white hat with embroideries, a long of "commonly used silk", and a hat covered in red velvet and topped by a small red tassel. He bore no staff of office. Earlier lists of offices, such as the appendix to the Hexabiblos, give slightly different ranks, placing him above the governor of a fortress and of the, and after the .