In general, exceptionalism is a doctrine that a particular entity is fundamentally different from others in its class. Use of the word frequently takes on a moral element, pasing a judgment on the fairness of making an exception from general rules.
The word may be used without a moral dimension to refer to the idea that a particular nation, region, person, situation, etc, constitutes a special case and cannot be used to predict or explain events elsewhere. However there is often an implied criticism involved in the usage of the word, an accusation that someone is guilty of either (a) hypocrisy, as when people claim special status for their own country, ethnic group, disability, religion, etc, or (b) a logical fallacy, as where someone attempts to circumvent a particular case which is inconvenient to their argument.
The most common use of the term is "American exceptionalism"1; this idea has two sides. Firstly, it is the belief that the USA is different and better from other nations in its small government, history of liberalism and belief in the free market, which are held to be certainly unique and probably meritorious. An example of this usage is:
Americans across the political spectrum embrace individualism, skepticism about government, and a faith in social mobility. Other developed nations, in contrast, have often embraced socialism and a strong state. Two new books document the advantages of this "American Exceptionalism."2
Secondly, there is the negative idea that the US claims a moral right to civilize, liberate or exploit the rest of the world, akin to the doctrine of "manifest destiny", as often with malign as beneficial consequences (from the genocide of the Native Americans to the USA's frequent decisions to support tyrannical governments or overthrow democracies if this would further US interests). Politically, exceptionalism becomes the belief that the USA considers itself unbound by the same strictures as other nations, and will submit to international law or the control of international institutions, while it still reserves the right to intervene in the rest of the world as it sees fit.3 (The British Empire appears to have operated according to similar justifications, and it would not be surprising if the Emperor Augustus thought likewise.)
However, I have seen many other uses of the word to refer to other alleged special cases, generally by authors questioning whether the entity under discussion should be treated differently from its peers.
"AIDS exceptionalism", the belief that AIDS is sufficiently different from other diseases in its nature, danger, prevalence, consequences, etc, that it should not be dealt with in the same way as other infectious diseases. The accusation of AIDS exceptionalism is generally levelled by those who believe HIV positive people are treated too well4, and can be seen as an attempt to portray AIDS activists as illogical or hypocritical.
"Gujarati exceptionalism"5, in an article attacking the belief that violence in Gujarat, India, in 2002 was the result of unique conditions which could not occur in the rest of India.
"Post-WWII Western European Exceptionalism"6, the idea that the success of a mixed economy in Europe following World War II was the result of special circumstances that mean the combination of state intervention and capitalism will probably not be successful elsewhere.
The doctrine has also been applied to Germany, South Africa, Cuba and many other nations.
1See for example Seymour Martin Lipset,
American Exceptionalism: A Double-Edged Sword (New York: W.W. Norton and Company, 1996) or, treating a more minor difference, Andrei S. Markovits and Steven L. Hellerman, Offside: Soccer and American Exceptionalism (Princeton University Press, 2001).
2The Cato Institute, "American Exceptionalism, Past and Future", <http://www.cato.org/events/000913bf.html> (14 January 2002).
3A typical attack on this version of American exceptionalism can be found in Hugo Young, "We can't allow US tantrums to scupper global justice", The Guardian, 2 July 2002, <http://www.guardian.co.uk/comment/story/0,3604,747686,00.html> (14 January 2002).
4Catherine Hanssens, "Inventing 'AIDS Exceptionalism'", The Body: An AIDS and HIV Information Resource, 1998, <http://www.thebody.com/hanssens/exceptionalism.html> (14 January 2002).
5Dipankar Gupta, "Gujarati exceptionalism", The Hindu, 16 May 2002, <http://www.hinduonnet.com/thehindu/2002/05/16/stories/2002051600391000.htm> (14 January 2002).
6J. Bradford DeLong, "Post-WWII Western European Exceptionalism: The Economic Dimension", 1997, <http://www.j-bradford-delong.net/Econ_Articles/ucla/ucla_marshall2.html> (14 January 2002).