Anything could be considered an empire. The mayor of a town could call all the 'lots' of his town seperate nations or fiefdoms or states and thus say that he, in ruling all the lots, is in fact an emperor.
It's similar for Japan. We think of Japan as a single entity. However, we're thinking with a modern eye on a modern map. Japan is a single entity because in the past an empire arose from squabbling warlords. The empire fused together disparate elements into Japan. It doesn't matter that we see Japan singly. What matters is the interpretation of ownership.
Boundaries are arbitrary.
Titles are arbitrary.
Emperors are emperors if they want to call themselves so.
However. As to the difference between emperors and dictators. Oxygen said it rather well. An emperor can be a dictator if he wants to. But doesn't have to be one if he doesn't.
However, she's (I think she's a girl. Right?) wrong in that a dictator can be an emperor if he wants to be one. After all, all he would need to do is dictate it to make it so.
The problem, of course, comes in on the outside. How one is perceived by those outside of your power. Castro can call himself an emperor all day and to his people, to those under his power, he would be one. But, those outside would laugh at his claims and call him a petty tyrant.
It's all about perceptions.
Opteronguy,
I have no idea what you're talking about. People have called America imperialist for time out of mind. We do come to an interesting point here, however. Now here we have a state of affairs where the detractors of a nation find pleasure in calling it imperialistic when that nation decries any imperialism.
The physical evidence is difficult to discern. We do have our fingers in quite a few pots and yet we have no nations that directly pay fealty to us. They don't pay us. In fact, to a large part, we pay them.
The power politics of today's world are far different than the times when empires arose. The modern era of politics is murky and no one has the courage to rise and say "By this axe I rule."