Okay, so you're going to need at least 80% of de jure territory, hold two kingdom titles or the specific empire title...no, that's not it. You'll need to do Shogun, then Medieval, then Rome, then Medieval II, and then you get Empire...no, not that, either.

I understand this is as much a topic of debate for the Capital-H Historians as it is for the entities that self-applied the label. 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.

I hate to make u/DanKensington feel like they've missed anything, but I have an answer that I wrote about whether or not the Soviet Union counts as an empire, to give you a sort of case-study-slash-example. And if anything in there is unclear, further questions are fine by me.