Augustus
Augustus' reign laid the foundations of a regime that lasted, in one form or another, for nearly fifteen hundred years through the ultimate decline of the Western Roman Empire and until the Fall of Constantinople in 1453. Both his adoptive surname, Caesar, and his title "Augustus" became the permanent titles of the rulers of the Roman Empire for fourteen centuries after his death, in use both at Old Rome and at New Rome. In many languages, "Caesar" became the word for "Emperor", as in the German "Kaiser" and in the Bulgarian and subsequently Russian "Tsar" (sometimes Csar or Czar). The cult of "Divus Augustus" continued until the state religion of the Empire was changed to Christianity in 391 by Theodosius I. Consequently, there are many excellent statues and busts of the first emperor. He had composed an account of his achievements, the "Res Gestae Divi Augusti", to be inscribed in bronze in front of his mausoleum. Copies of the text were inscribed throughout the Empire upon his death. The inscriptions in Latin featured translations in Greek beside it, and were inscribed on many public edifices, such as the temple in Ankara dubbed the "Monumentum Ancyranum", called the "queen of inscriptions" by historian Theodor Mommsen.
Emperor
When Republican Rome turned into a "de facto" monarchy in the second half of the 1st century BC, at first there was no name for the title of the new type of monarch. Ancient Romans abhorred the name Rex ("king"), and it was critical to the political order to maintain the forms and pretenses of republican rule. Julius Caesar had been Dictator, an acknowledged and traditional office in Republican Rome. Caesar was not the first to hold it, but following his assassination the term was abhorred in Rome.
Augustus, considered the first Roman emperor, established his hegemony by collecting on himself offices, titles, and honours of Republican Rome that had traditionally been distributed to different people, concentrating what had been distributed power in one man. One of these offices was "princeps senatus", ("first man of the Senate") and became changed into Augustus' chief honorific, "princeps civitatis" ("first citizen") from which the modern English word and title prince is descended. The first period of the Roman Empire, from 27 BC – 284 AD, is called the "principate" for this reason. However, it was the informal descriptive of "Imperator" ("commander") that became the title increasingly favored by his successors. Previously bestowed on high officials and military commanders who had "imperium", Augustus reserved it exclusively to himself as the ultimate holder of all "imperium". ("Imperium" is Latin for the authority to command, one of a various types of authority delineated in Roman political thought.)