Stupid keywords. This is what happens when you start obsessing about stats and traffic. You can thank our best friends, the search engines, for this post.
I noticed that people land on my articles searching for “Roman Empire”, in various combinations. Very few look for “Roman Republic” and nobody uses “Ancient Rome”, “Roman Kingdom”, “Roman Principate” or “Roman Dominate”. But technically speaking, and from a very geeky point of view, there was no Roman Empire.
So, here's the thing. Rome started out as a kingdom. The years conventionally accepted are 753 BC – 509 BC (stress here goes on conventionally). The kingdom was ruled by, oh, well, a king (who most likely wasn't Roman, by the way, but that's a different story).
508 BC – 27 BC is the period of the Roman Republic. The highest ordinary magistrate during this period was the consul. Two of them were elected for one year term.
In 27 BC, the Republic was “replaced” by the Principate, which lasted until 284 AD. This means that, officially, nothing changed from the republican times. The Senate was still there. The consuls were still elected, two every year (up to twelve per year, actually, for various reasons). All forms of the Republic were kept in place. Augustus – now credited as being the first emperor – went to great lengths to avoid setting in place any definite mechanism to change the ways of the republic.
Basically, Augustus watched what happened to his adoptive father, Julius Caesar, and considered he didn't need to piss anybody off to that extent. There were periods of time when he held no official function. He ruled based on his personal authority and respect he got from others, and consistently referred to himself as “first among the equals” (princeps – hence, the Principate).
Princeps senatus meant the first of the senators – as in the first senator to speak during a session. It may not seem much today, but it was enough to allow Augustus an effective control over the Senate – he could make his point known from the beginning, and nobody dared contradict him.
Augustus controlled Rome through a very complex mechanism, which depended a lot on his personality. Trouble started when his heirs could no longer leverage the same mechanisms. There was also the very tricky issue of the transfer of power, which was not regulated, since there was no official function to be handed down from one generation to another.
Augustus did not call himself “Emperor”, in the sense we use this word today. The equivalent at the time would've been identical to “king”, and that was exactly what he was trying to avoid. But the Latin word “imperator” was commonly used for high-ranking military leaders – any given generation could have as may imperators as needed.
This complicated and highly sensitive form of government came to an end in 284 AD, when Diocletian started officially referring to himself as “dominus” (meaning “master” or “lord”) which buried the hypocritical claim that the Republic was still alive.
Now, don't get me wrong. I'm not saying it's wrong to use “Roman Empire”. Just that it's weird that's all people search for.