Considering that an “empire” in the political sense is a lot like a federation, except with involuntary membership of the member states and peoples, I’d say no, the US is not an empire as such. We don’t exert coercive hegemonic power over our member states.
Back around 1900-1940 or so, we were somewhat imperial, with the Spanish-American war changes in possession of Cuba, Puerto Rico, the Phillipines and a few other areas, but now the only parts of the US that are not states are Puerto Rico and a very few Pacific island possessions claimed during World War II. A fairly pitiful empire by any standard.
We are a sort of soft-power empire though, in the sense that our economic might allows us to dictate a great many things to the rest of the world, and especially to our close economic neighbors. This is mostly in the realm of trade, although sometimes this power is used to coerce certain human rights policies or other policies, but that’s more the exception than the rule.
I guess the main distinguishing thing is that Barack Obama can’t just issue an executive order that Mexico must make their water safe to drink, and President Nieto has to comply or face US troops. Instead, if safe Mexican water was important, we’d have to take some sort of circuitous indirect route to try and carrot and stick them through economic means into having safe water. That’s not how classical empires have ever worked.
The conquest of North America wasn’t imperialistic- there wasn’t any notion of including existing Native American states in our framework a-la the Romans, British or Macedonians under Alexander. That would have been imperialistic, but merely driving the Native Americans out is pretty much anti-imperialistic.