Julius Caesar was the last Roman dictator. After his assassination, no Roman ever again used that title, which in the Western tradition soon (and unfairly) adopted similar undertones to the Greek “tyrant.”

Augustus’ accession to the pseudo-Hellenistic hereditary post of first man and protector of the Roman Republic marked the start of the Roman Empire proper, given that his successors — obviously — preferred the neutral title “Emperor” over any other of those they inherited from the first such emperor.

“Imperator” roughly translates as “great commander” since one with “imperium” had “command,” as authorized by the senate and the Roman people. So the Roman Empire (Imperium) itself was, from this point of view, a bit of an euphemism.