An autocrat is generally speaking any ruler with absolute power; the term is now usually used in a negative sense (cf. despot and tyrant). The term is derived from the Greek word autokratôr (lit. "self-ruler", "ruler of one's self").
The principal titles of what modern historians call the "Roman emperors" were imperator, Caesar, and Augustus; the latter two words were transliterated into Greek as kaisar and augoustos, while the existing word autokratôr was substituted for the former. When the Emperor Heraclius introduced the "Byzantine" system of co-emperors, the senior emperor (or, in the absence of a co-emperor, the sole emperor) took the title autokratôr, although the junior emperor also began to take that title in the 14th century under the Palaeologi.
In keeping with the contention of the rulers of Russia that Moscow was "Third Rome" (after Constantinople and Rome), the formal title of the Russian emperor was Imperator i Samodyerzhets Vserossiysky ("All-Russian Emperor and Autocrat"). The absolutist rule of the Russian emperors is probably chiefly responsible for the modern negative connotation attached to the word "autocrat".
See also: Roman Emperors; List of Roman Emperors; Byzantine Emperors; List of Byzantine Emperors; Tsar