The word "empire" has many definitions, however I feel like Brain4Breakfast's in "What the Gulf? - Nation Building" is the best;
An empire is a group of distinct people, called nations, all ruled over by one, in an asymmetric relationship.
However, there are other definitions of empire, which are different from B4B's definition in other dictionaries and by other creators, notably Wikipedia's who says nothing about nations but about ethnic groups:
An empire is a multi-ethnic or multinational state with political and/or military dominion of populations who are culturally and ethnically distinct from the imperial (ruling) ethnic group and its culture.
With these different definitions of empires and definitions of empire itself, this begs the eternal question: is the United States of America an empire?
Personally, I don't think the US is an empire. It may be imperialist, but that doesn't make it an empire; an empire needs a figurehead or a powerful, sovereign leader (especially a monarch) that can be seen as an embodiment of the state in my opinion. The US President doesn't fit that role; he's certainly not as powerful as an emperor and he's not a figurehead like Hitler or Caesar.
What are your opinions, NSG?