My understanding is that and empire is an authoritarian domination of several ethnic groups but it seems like the definition is not consistently applied. Were the Chinese dynasties really empires? Were the Sumerians or Egyptians? In the more modern context I think I understand it better especially when it references the British, French and Germans because it feels more exploitative but I am always left scratching by head wondering what splits the difference.
Oooof. This is one of Those Questions where the answer is just so dang annoyingly fuzzy. I can, however, state that the dividing line is 80% of de jure territory and either two kingdom titles or the specific empire title.
Wait, no, that's the CK2 qualifications. The thing is, the fuzziness means that this is much contested ground up there in academia-land. More can always be said as to who gets the E-word applied to them, so if anyone would like to address themselves to an answer, please don't let this post stop you!
For the meantime, OP, I commend to your attention u/b1uepenguin and u/AlviseFalier both ruminating on just what makes Empire. Further, u/mikitacurve gives us as a potential case study whether or not the Soviet Union counts as an empire.