I prefer to stick to simple definitions and the US is not an empire in my mind. But it is interesting to see how many people on this site don't mind the term at all..one lives and learns. That is why its always good to ask. One can find out a lot by just asking...
The way I usually hear the term used is in postmodern/postcolonial discourse (for some reason, the South Asian upper classes are very heavily invested in this concoction..much more so that the Chinese or the Vietnamese..I have some vague theories about why that may be so). There, it is taken for granted that the US is an empire..in fact it is "the empire" without need for further qualification. I find it very irritating, not because I approve of US foreign policy or all its interventions around the world, but because I find it to be a remarkably useless term; misleading and so far from reality that anyone using it as their main unit of analysis is bound to hit a brick wall very soon....