I've been pondering this question a lot lately, and I think a lot of it actually boils down to identification as such, especially if you're taking the slightly post-modern tack of a word's definition as not something fixed, but as a negotiated social construction.

For example, for non-self designations, you have the idea of a structural empire. Meaning a political entity that may not consider itself an empire, or may not actually have that exact political construction in mind, but that we as modern scholars label as such because it follows the characteristic "features" of an empire. Good examples would be the Athenian Empire, which were greek city states under the leadership of Athens. However, they did not call themselves the Athenian Empire, they called themselves the Delian League. WE just happen to call it an empire because it matches the description, of which one (because there are many) would be "a geographically extensive group of states and ethnic group) united and ruled either by a monarch or an oligarchy" according to Wikipedia.

However, rest assured this is not the only potential definition, because quite frankly, it is the definition itself that creates the problem of what exactly an empire is.

Moving from polities that have not self-identified as empires to ones that do, we have an entity like the Korean Empire, which called itself as such in order to see itself as political equals to the Chinese Empire and other foreign powers. However, it was not a far flung geographical state ruling over multiethnic groups. Most people would probably put the "empire" in Korean Empire in quotes. However, if they are self identifying as an empire, who exactly are we to say that they're not?

Because this ultimately relates to the problems of the "demise" of the Roman Empire, and the "survival" of the Chinese Empire. I've actually discussed this quite frequently in recent posts comparing China and Rome, but it probably is worth mentioning here as well.

Rome and China represent flip sides of the question of empire and imperial continuity. Rome could be argued to represent structural though not self-identification continuity, where as China would be the opposite.

Though the denizens of the former Roman Empire stopped calling themselves Roman by the 6th century (except by the steadily dwindling areas around Constantinople), many of the structural frameworks of their empire continued in successor states like the islamic caliphate, or the carolingian empire, who alternatively used imperial rhetoric or ceremony, or in the caliphate's case, continued infrastructure. And yet neither called themselves Roman (even with Charlemagne, the Roman in their Roman Empire was geographical, not ethnic). So is it the same empire?

On the flipside, you have the Chinese Empire, which is so often viewed as monolithic, but in reality the variance in political and bureaucratic structure between each successive dynasty could be as wide as the differences between the late roman empire and the kingdom of france, not to mention the ethnic composition of their leadership class would be just as wide, between rule by han, or various barbarian tribes. Yet we call them a cohesive and continuous empire. Why? Because in this case, it's their own self-identification. Although with that said, their self identification is still coterminous with a structural definition, but that structure changes with each successive dynasty. So is it really the same?

All of which boils down to my core point. That what makes an empire an empire is quite nebulous, and is in fact a combination of any degree (including in some cases no degree) of adherence to a structural definition and recognition as such by either the state itself or others.

Put simply, what makes an empire an empire is based on whoever allows or wants that political entity to be called an empire, and the people choosing to call a political entity an empire (or not) don't have to agree.

Confusing? Yes. But then again, so is empire, however it's defined.

I wonder what the different answers you've gotten are, because it is quite straightforward. An empire is a group of states that are all under a single authority's rule. Rome ruled several nations throughout the Mediterranean such as Egypt, Greece, Spain, etc. Britain had many states under its rule all over the world. The HRE was a group of germanic states who had an emperor in charge of them all.

Source: Oxford dictionary and any dictionary