Following Caesar’s assassination, the Roman Republic, which had, in effect, been dead for some time, morphed into the Roman Empire. At what time did the Romans themselves start referring to it as the Roman Empire? And if, in fact, they did not, how did they refer to it in, say 350 A.D.?
I would assume on the inaugeration of the first Emperor, Augustus.
They continued to refer to it as the res publica, which by that time meant ‘the state’. Much the same as the British continuing to refer to Britain as ‘the kingdom’ when, for pretty much all practical purposes, the King is powerless.
imperium referred, originally, to a general’s power delegated to him by the Senate to raise troops and carry out military operations. Gradually it changed to refer to the power and position of the emperor. Such changes typically happen over a very extended period, and would not have been complete by the death of Augustus.
Augustus upheld the appearance of republican government for decades. He was merely a consul who was granted (temporary) extra powers by the Senate. While the beginning of the Empire has to logically be dated to sometime within his reign, I’m not sure I’d date it much before he named his successor.
Firstly I’ll have to give my regular plug for the absolutely awesome History of Rome podcast that covers this in some detail (and despite that is very interesting and listenable).
As far as I’m aware the illusion of republican government was maintained for long in the imperial period (possible throughout it). Earlier emperors (such as Augustus) spent a deal of effort maintaining the illusion, most did not even pay lip service to the idea.
The term emperor (or Imperator) was a military title that predated the Imperium (so “emperors” that had no military experience would never have refered to as such in the early Imperium at least).
As others have noted, the actual position of “Emperor” did not exist in the early to middle Empire. One became emperor by accumulating a number of Republican (in origin) positions. Republican form and even the idea that the Republic continued was adhered to for a very long time.
+1 on that podcast, and combine it with a listen [or read] of Lives of the Caesers.
Historians in that era were more publicists … they would praise the current in power and tell great scandalous gossip about the opposition. Facts were only incidental 
I understand all this and I appreciate the comments made by earlier posters, but I am not asking at what point it would be proper for us to speak of “The Roman Empire” instead of “The Roman Republic”, but rather when did the Romans themselves begin to refer to their state as “The Empire.”
I doubt they ever did. For instance, opening at random the Scriptores Historiae Augustae, probably compiled in the late 4th c. but ostensibly much earlier, we read in Tacitus, II:-
*Ergo, quod rarum et difficile fuit, senatus populesque Romanus est ut imperatorem per sex menses, dum bonus quaeritur, res publica non haberet. * (and so the senate and people of Rome passed through an unusual and a difficult situation, namely that for six months, while a good man was being sought, the state had no emperor)
First, I am definitely not a Latin scholar! Maybe one will turn up soon. Having said that I think the problem is, as Mk VII has pointed out “Imperium” did not mean empire as we know it. Imperium Romanum was the term used to cover the area subject to Roman rule, not a political system, and it was used during the Republic, the early Empire, and the later Empire.
“Emperor” was never the official title of the ruler of the “Roman Empire”. Early Emperors were know as *princeps *(‘First’), later ones as *dominus *(“Lord”), and still later as *Basilus *(Greek for king). They did have *imperator *in their long list of titles but it had more of a military style than a consitutional role. From Diocletian on the key title for Emperors was Augustus with Caeser the title for their nominated successor. See the Wiki entry for Roman Emperorfor more on this.
So, as far as I can see, the answer to the question "when did the Romans themselves begin to refer to their state as “The Empire?” is “Never”.
Missed the edit window! I should learn to Preview - beaten to it by **Mk VII ** only he can actually translate latin 