It also influences other countries to an extent which is similar to other empires like the British did. It invades countries to keep trade going, and it's companies dominate the world much like the companies of the British empire did.

The only difference is that the US has finished expanding.

So what really constitutes an empire? Does it need an emperor? Does it need to constantly be questing to expand in lands?

War: True, the US engages in war, and the British Empire engaged in war. But so have many countries that were no empires. When the US engages in war, it leaves and tries to set up a gov't as the new authority. This has worked well (Japan, Germany, etc.) and not so well (Iraq, South Korea initially).

Companies: Imperial British companies literally captured their markets -- they had their own armies! The British East India Company for example came to rule much of the subcontinent, with their own soldiers! Subjects of British imperial rule were told what to trade. They might have done well for themselves sometimes (or been devastated at other times), but in any case the relationship was clear.

Global leadership: The US took the mantle as global leader from the British. This wasn't desired. Indeed after WWI the US refused to play leader. Some historians believe that WWII could have been avoided if the US would have fulfilled its duty as most powerful to provide leadership.

After WWII the US became serious about being a leader, by which I mean taking the initiative on issues of global coordination, both security-wise and financial. This is another reason the US is so easily confused with an empire. The UN, IMF, World Bank, NATO, Bretton Woods, these were largely American initiatives, but they were not imperial dictats.

Again, the importance in my way of thinking is that countries with relationships with the US are not subjects of the US. That is the key distinction that must never be forgotten.

A question: In your way of thinking, is the EU an empire?

All other uses of the term are metaphorical. We can talk about the McDonald's hamburger empire.

When we talk about the American Empire, we're also using the term metaphorically, because think what you will of the US, but it doesn't rule as a supreme authority over anything but a few islands. It may have friends, allies, interests, influence, bargaining power, but it has no subjects, no subjugated peoples. It may have largely founded what is now the world's economic and security order after WWII, but those are all voluntary.

The problem is that the overlap between superpower and empire makes the term empire confusing in the American case. "Powerful like an empire" and "an empire" are very different.

When we talk or think about the "American Empire", we seek similarities and ignore differences with the Roman Empire, European colonial empires, even the Athenian Empire or Soviet Empire. It makes it sound like the US can tell other countries what to do as it pleases, that the US rules over others. That's a very important mistake, even a dangerous mistake. Allies are not subjects. When a country asks the US military to leave, it leaves. See the Philippines as a good example (and further back as an example of the bad old days, when the US was briefly interested in having a real empire).

Today's US is an empire in the sense that Microsoft is an empire -- it is an empire of the willing who can leave whenever they like.

And if you can leave whenever you'd like, it's not a real empire.