This is one of those topics that drives me nuts. The definitions typically are pretty clear. An empire consists of a centralized government (or not centralized, that can be debated) in which Kings and or Kingdoms are vassals to a single individual (obviously it can be debated that governors can be included). A kingdom has no vassals (again the semantics can be argued). As a dealer I often wonder these things as I wish to present coins in the most honest manner, but I have to guess at the disparity one encounters with coin descriptions. Why is there a Roman Empire but not a Macedonian Empire? They operated essentially the same and under similar structure. Why are the Seleukids regarded as a 'Kingdom' bust just as often an 'Empire'?. I have no doubt some scholars might dismiss my question as being ignorant, but if one examines history it can become a bit confusing. It can be postulated that the United States (and nearly all other countries in the world) are also 'Empires'.
I think "kingdom" implies a certain amount of cultural homogeneity and "empire" is meant to signify a military and administrative organization that controls other, previously established, cultures. Thus Macedonia was a kingdom after it's borders shrank back down to the Hellenic areas it controlled and the Seleucid's were an empire in the same vein as the Persians (because they controlled Babylonia, Syria, Pars etc.). At least that's how I think of it.
This is a good example of why people wonder. Alexander did more to spread Hellenism (though he was not a Greek and as such opens up a whole new set of questions) than any other person is history, and he was just an incredibly simple and obscure character who through various mishaps and simple luck became one of the most respected military commanders of all time. So why was he relegated being a simple 'Macedinian King' and not the first 'Macedonian Emperor'?.
