My friend Sydney Freedberg and I recently had an exchange on his website about the definition of the word "empire," and whether it properly applied to the United States. (This was apropos of an interview Sydney did with a Romanian news site on the NATO campaign in Libya.) Sydney defines an empire as a state with a "politically dominant, culturally distinct group" living in a core territory, and at least one ethnically distinct peripheral group with limited political rights. By this definition, the U.S. was an actual empire from 1898 to 1946, during its period of formal rule over the Philippines and establishment of protectorates over several Caribbean and Central American republics.
I think it's useful to have a limited definition of empire, if only because the term has become so widely and pejoratively used in the early twenty-first century as to lose its meaning. I would only add two caveats here. The first is that "empire" wasn't always pejorative; during the eighteenth century, for instance, it could merely mean "a large territorial state." As Peter Onuf points out in Jefferson's Empire (U. of Virginia Press, 2001, pp. 53-79) the first leaders of the American national republic frequently referred to the United States as an "empire." Indeed, Federalists discovered it was more politically useful to call the U.S. an "empire" than a "nation," since the latter implied that they wanted to create a consolidated national government (as their Anti-Federalist critics claimed).
The second caveat is that under Sydney's definition, the interstellar Empire in the original STAR WARS movies wasn't an empire, unless you count the Stormtroopers as the "core" ethnic group. Pretty much everyone else was an oppressed peripheral group. (In the prequel films, there was only one consistently oppressed subject race: the audience.)