I suppose extraterritorial aggression, coupled with complete hegemony over the lands conquered like Phid mentioned, and an autocratic/authoritarian regime as the central government.  Rome was an oddity because it assmimilated its subjects while also assimilating some of their cultural nuances at the same time.  Rome was belligerent but tolerant of long standing cultural traditions as well.  Rome was also a confederation of Latin city-states in the beginning that was held together under by the perpetual use of war….the most potent of unifiers.  But I think the most important thing that qualifies a kingdom or state as an “empire” would be the imposition of its laws upon the lands it conquers.  Hammurabi gave the first codified set of laws for the NeoBabylonian Empire, and Justinian did the same for the later Roman Empire.  Napoleon offered his law codex as well.  Hitler created national socialism in a similar vein.  Lenin gave Russia Bolshevism and the new order that resulted from its precepts.  I think we could call all of these examples true empires.  But just like anything else, no one particular mold covers them all.  Athens and Sparta were both empires but completely polar opposites.  Medo-Persia was completely different from the Greeks or the Romans.  Great Britain's empire was a commercial enterprise and based on commonwealthman common laws.  America could be perceived as an empire via its Manifest Destiny yet it is not autocratic….yet it did redefine representational democracy and individual freedom.  So my estimation is that imperial enterprises come in many shapes and sizes…forms and fashions.  So there is not any “litmus test” that can universally label a nation an empire. JMHO

Conquering with Superior might is certainly a prerequisite. I like Donald's point about a set of laws put in place for the conquered. What about what a country can accomplish with the development of weaponry and technology? If they are on the leading edge I'd say it contributes. Or Architecture and art? Literature and philosophy? If we get beyond war and military, how do those things contribute?

I am going to have to disagree with some of this. A classic example to the contrary being the Holy Roman Empire. The Emperor was decided by a imperial diet of prominent Electors and could not actually call himself Emperor until he had travelled to Rome to be personnally crowned by the Pope. The HRE for much of its existance was therefore more a confederation of sovereign States and therefore the submission that an Empire must involve 'conqueror' and 'conquered' simply does not hold water.

Couldn't the present European Union be also called an Empire (of sorts) with a European Parliament and a President (Emperor!) elected by the Sovereign States who still retain their own Head of States and Governments.Much like the Holy Roman Empire (which has nothing to do with the Roman Empire of the Caesars era) was during an earlier time.

I disagree. The fact that the Pope personally crowned the Emperor does not in and of itself make the Holy Roman Empire, an Empire. I might be wrong, but by that time Rome wasnt what it used to be. And those soverign states? How did they become part of the Roman empire?

The pope was able to give legal title “emperor”, even if it was at times lacking the normal connotations of what an emperor is all about.  For example, I think the crowning of Henry I (Henry the Fowler) as HRE in the 10th century did not make him an “emperor” as we might otherwise think of it.