If imperial has any meaning in the post-colonial 21st century, it certainly means that the (super)power in question has an active interest in attempting to control significant swathes of the planet. In fact, there has never been a power, no matter how "great," that has, in such a militarized way, tried to put its stamp of control on so much of Planet Earth.
The much-vaunted "liberal order" policed by the U.S. was a product of World War II and the Cold War. Germany and Japan had to be kept down, the Communist powers had to be contained, and the old countries of Europe had to learn to live with one another under unifying pan-national institutions. All of this was made possible by American money and military might. As a result, the Free World, in Western Europe and East Asia, became a US dependency. This cannot go on forever. Indeed, the arrangements are already fraying. But then comes the old imperial paradox. The longer others remain dependent on the U.S., the less capable they will be of taking care of their own affairs, including their security. And, like an authoritarian parent, the U.S. itself, despite its admonitions to its allies to pull their weight, is often loath to let go of its increasingly unruly dependents.