The Dominate was the "despotic" latter phase of government in the ancient Roman Empire from the conclusion of the Third Century Crisis of 235-284 until the formal date of the collapse of the Western Empire in AD 476. It followed the period known as the Principate. In the Eastern half of the Empire, and especially from the time of Justinian I, the system of the Dominate evolved into Byzantine absolutism.

it is derived from the Latin dominus, meaning lord or master, an owner versus his slave - this had been used sycophantically to address emperors from the Julio-Claudian (first) dynasty on, but not used by them as a style - Tiberius in particular is said to have reviled it openly. It became common under Diocletian, who is therefore a logical choice as the first ruler of the "early" dominate. Historian David Potter describes the transformation of government under Diocletian when describing the shifts in imagery the Emperor used to display his power (in this case the building of a huge new palace at Sirmium).

During the Principate - the first period of the Roman Empire - the formalities of the constitutionally-never-abolished Roman Republic were still very much the "politically correct" image of Imperial government. It has also often been said to have ended after the 235-284 ADCrisis of the Third Century, which concluded when Diocletian established himself as Emperor. Moving the notion of the Emperor away from the republican forms of the Empire's first three centuries, Diocletian introduced a novel system of joint rule by four monarchs.

The Tetrarchy, and he and his colleagues and his successors (in two imperial territories, east and west, not four) chose to stop using the title princeps. Instead, they openly displayed the naked face of Imperial power, adopting a Hellenistic style of government more influenced by the veneration of the Eastern potentates of ancient Egypt and Persia than by the heritage of civic collegiality amongst the Roman governing class passed down from the days of the "uncrowned" Roman Republic.

Emperors of the Principate, emulating Augustus in his fiction of a republican government, created the idea of the Emperor as a concentration of the various civil and military offices upon one individual, nevertheless hiding any autocratic or despotic connotations by the preservation of the Senate and other facets of the Republican period, such as the annual paired consulship. After Diocletian, however, Emperors started to wear jeweled robes and shoes, in contrast with the simple toga praetexta used by Principate Emperors in emulation of Augustus.

Emperors inhabited luxurious palaces (the ruins of Diocletian's enormous palace in Dalmatia survive to this day; see Diocletian's Palace) and were surrounded by a court of individuals who, only due to the favor and proximity of the Emperor, attained the highest honorific titles and bureaucratic functions. In fact, many offices associated with the palatine life and that suggested intimate relationship with royalty eventually developed connotations of power, such as the offices of Chamberlain and Constable. The titles of Senator and Consul, after the loss of every residue of political power they had had in the Principate, became an honorific in the later Empire.

The adoption of Dominus as a formal title reflected the divine status (divus) that has come to be a prerogative of the Imperial position. Originally an exceptional honor awarded by the Senate to an Emperor posthumously, the elevation had devolved to an expected convention for still living Caesars. To dissuade the rebellions and usurpations of the Crisis of the Third Century, the Emperors sought the kind of divine legitimacy invoked by Eastern monarchies.

Emperors imported rituals such as kneeling before the Emperor, and kissing of the hem of the Imperial robe (proskynesis). After the personal adoption of Christianity by Constantine I (312), and its installation as the official state religion by Theodosius in 380, Imperial divinity became directly associated with the Christian Church. In the Eastern Roman Empire after AD 476, the symbiotic relation between the Imperial Crown in Constantinople and the Orthodox Church led to the distinctive character of the medieval Byzantine state.

Another clear symptom of the upgrading of the imperial status was the notion of the emperor as an incarnation of the majesty of Rome; thus lese majeste became high treason.

Present historians reject the interpretation of the transition from Principate to Dominate as a clear, easily definable break (cf. Late Antiquity). Rather, they now characterize it as a much more subtle, gradual transformation, in which Diocletian's reforms of the Imperial office, while significant, are but one point on a sliding scale.the distinction between two primary phases of Imperial government in Rome remains an important and useful one.