American exceptionalism (by the way, physicsforums doesn't recognize "exceptionalism" as a word) is the belief that America and it's traditions of government are historically unique. I.E., the American "empire" will not go the way of other empires because it is, in some sense, categorically different.
I think the first sentence from the above referenced wiki article better answers the OP's question:
"American exceptionalism refers to the theory that the United States is qualitatively different from other countries. In this view, America's exceptionalism stems from its emergence from a revolution, becoming "the first new nation,"[1] and developing a uniquely American ideology, based on liberty, egalitarianism, individualism, populism and laissez-faire. This observation can be traced to Alexis de Tocqueville, the first writer to describe the United States as "exceptional" in 1831 and 1840.[2] Historian Gordon Wood has argued, "Our beliefs in liberty, equality, constitutionalism, and the well-being of ordinary people came out of the Revolutionary era. So too did our idea that we Americans are a special people with a special destiny to lead the world toward liberty and democracy."