Quote Originally Posted by Cyclops
Empire is related to the latin term Imperator, one who conquers. I'd say an Empire is that which is conquered, as opposed to that which is truly equal or otherwise associated.
Just to nitpick here, Imperator is "one who commands" and is a honorary title for a general awarded almost spontaneously by his troops after a victory on the battlefield. It doesn't even mean "emperor" as we use the word today. The Roman emperor's official title was "president of the senate" (princeps senatus), which was a honorary rank already existent in the Republic.

Linguistics won't get you anywhere on this, nor will anything else because the term "empire" is ill defined; at best it describes a relation of power as Watchman said, but that leads to "empire" meaning nothing in absolute terms, it could just as well be called "hegemony" or anything. Putting the "Athenian Empire" next to the "Roman Empire" on the list, for example, shows that when one speaks of empire it doesn't describe a power in absolute terms (the disparity is obvious). In that way you can find an "empire" under any rock you lift, so I don't know what use the term is. Perhaps it is my Greek linguistic background, but when in Greek we refer to a state entity as "autokratoria (=empire)" we generally mean it's humongous (British empire, Roman empire, Ottoman empire e.g.) in terms of military power, expanse of land and multitude of nations it rules over; nobody would call in Greek the Athenian hegemony an "autokratoria" for example, and the modern cavalier use of "empire" in English leaves me somewhat perplexed.