American exceptionalism is a term used to describe the idea that America differs from other nations because of its unique history and the fact that it is the only nation founded on a creed: The Declaration of Independence. This creed revolves around the idea of what makes a good society. It is our sense that our nation's focus is to create a good society based on personal and economic freedom for its citizens and hope for the rest of humanity.
American exceptionalism implies that we are different from European countries, despite their relationship to our genesis, and from other countries in the world. It does not necessarily imply superiority, but more a sense of the possibilities of a better world.
Critics point out that the idea of American exceptionalism has been used as a justification for the U.S. to promote its ideals and way of life in other parts of the world. It is sometimes compared to the Manifest Destiny that marked the extension of our borders, via expansionism and the imperialism which followed.
I would argue, first of all, that many countries that do not claim to be exceptional have been imperialistic. In times past, most of the countries of Europe have been imperialistic. So has Japan. You can argue that China has been imperialistic in the past as well. The point here is that it is really power that makes countries imperialistic, not ideologies. Therefore, I would argue that it is not the idea of American exceptionalism that leads to imperialism. Instead, imperialism is something that has been and is done by every country that has had sufficient power to be imperialistic.