In the late 3rd century, after the Crisis of the Third Century, Diocletian formalised and embellished the recent manner of imperial rule, establishing the so-called Dominate period of the Roman Empire. This was characterised by the explicit increase of authority in the person of the Emperor, and the use of the style Dominus Noster ("Our Lord"). The rise of powerful Barbarian tribes along the borders of the empire and the challenge they posed to defense of far-flung borders and unstable imperial succession led Diocletian to experiment with sharing imperial titles and responsibilities among several individuals - a partial reversion to pre-Augustian Roman traditions. For nearly two centuries thereafter there was often more than one emperor at a time, frequently dividing the administration of the vast territories between them. As Henry Moss warned, "Yet it is important to remember that in the eyes of contemporaries the Empire was still one and indivisible. It is false to the ideas of this time to speak of 'the Eastern and Western Empire'; the two halves of Empire were thought of as 'the Eastern, or Western parts' (partes orientis vel occidentis.)" However, after the death of Theodosius I in 395, the split became firmly entrenched (see: the Western Roman Empire and the Eastern Roman Empire). The last pretense of such division was formally ended by Zeno after the death of Julius Nepos in 480. For the remaining thousand years of the Eastern Roman Empire there would only ever be one legitimate senior emperor, ruling from Constantinople and maintaining claim to the increasingly unstable territories in the west. After 480, multiple claims to be the imperial title of Augustus (or Basileus for Greek speakers) necessarily meant civil war, although the experiment with designating junior emperors (known as Caesars), usually to indicate the intended successor, occasionally reappeared.
The Empire and chain of emperors continued until the death of Constantine XI Palaiologos and the capture of Constantinople by the Ottoman Empire in 1453. The use of the terms "Byzantium," "Byzantine Empire," and "Byzantine Emperor" to refer to the medieval period of the Roman Empire has been common, but not universal, among Western scholars since the 18th century, and continues to be a subject of specialist debate today.
The emperors listed in this article are those generally agreed to have been 'legitimate' emperors, and who appear in published regnal lists. The word 'legitimate' is used by most authors, but usually without clear definition, perhaps not surprisingly, since the emperorship was itself rather vaguely defined legally. In Augustus' original formulation, the princeps was selected by either the Senate or "the people" of Rome, but quite quickly the legions became an acknowledged stand-in for "the people." A person could be proclaimed as emperor by their troops or by "the mob" in the street, but in theory needed to be confirmed by the Senate. The coercion that frequently resulted was implied in this formulation. Furthermore, a sitting emperor was empowered to name a successor and take him on as apprentice in government and in that case the Senate had no role to play, although it sometimes did when a successor lacked the power to inhibit bids by rival claimants. By the medieval (or "Byzantine") period, the very definition of the Senate became vague as well, adding to the complication.
Lists of legitimate emperors are therefore partly influenced by the subjective views of those compiling them, and also partly by historical convention. Many of the 'legitimate' emperors listed here acceded to the position by usurpation, and many 'illegitimate' claimants had a legitimate claim to the position. Historically, the following criteria have been used to derive emperor lists:
So for instance, Aurelian, though acceding to the throne by usurpation, was the sole and undisputed monarch between 270–275 AD, and thus was a legitimate emperor. Gallienus, though not in control of the whole Empire, and plagued by other claimants, was the legitimate heir of (the legitimate emperor) Valerian. Claudius Gothicus, though acceding illegally, and not in control of the whole Empire, was the only claimant accepted by the Senate, and thus, for his reign, was the legitimate emperor. Equally, during the Year of the Four Emperors, all claimants, though not undisputed, were at some point accepted by the Senate and are thus included; conversely, during the Year of the Five Emperors neither Pescennius Niger nor Clodius Albinus were accepted by the Senate, and are thus not included. There are a few examples where individuals were made co-emperor, but never wielded power in their own right (typically the child of an emperor); these emperors are legitimate, but are not included in regnal lists, and in this article are listed together with the 'senior' emperor.