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The term nostalgia describes a sentimentality for the past, typically for a period or place with happy personal associations. The word is a learned formation of a Greek compound, consisting of νόστος (nóstos), meaning "homecoming", a Homeric word, and ἄλγος (álgos), meaning "pain, ache", and was coined by a 17th-century medical student to describe the anxieties displayed by Swiss mercenaries fighting away from home. Described as a medical condition—a form of melancholy—in the Early Modern period, it became an important trope in Romanticism.
In common, less clinical usage, nostalgia can refer to a general interest in the past, their personalities, and events, especially the "good old days" from one's earlier life. Boym argues that nostalgia is more prevalent during times of great upheaval.
The scientific literature on nostalgia is quite thin, but a few studies have attempted to pin down the its essence and causes. Smell and touch are strong evokers of nostalgia due to the processing of these stimuli first passing through the amygdala, the emotional seat of the brain. These recollections of our past are usually important events, people we care about, and places where we have spent time. Music and weather can also be a strong trigger of nostalgia.
As a medical condition 
The term was coined in 1688 by Johannes Hofer (1669–1752) in his Basel dissertation. Hofer introduced nostalgia or mal du pays "homesickness" for the condition also known as mal du Suisse "Swiss illness" or Schweizerheimweh "Swiss homesickness," because of its frequent occurrence in Swiss mercenaries who in the plains of lowlands of France or Italy were pining for their native mountain landscapes. Symptoms were also thought to include fainting, high fever, indigestion, stomach pain, and death.
English homesickness is a loan translation of nostalgia. Sir Banks Joseph used the word in his journal during the first voyage of Captain Cook. On 3 September 1770 he stated that the sailors "were now pretty far gone with the longing for home which the Physicians have gone so far as to esteem a disease under the name of Nostalgia," but his journal was not published in his lifetime (see Beaglehole, J. C. (ed.). The Endeavour Journal of Joseph Banks 1768-1771, Public Library of New South Wales/Angus and Robertson, Sydney, 1962, vol. ii, p. 145). Cases resulting in death were known and soldiers were sometimes successfully treated by being discharged and sent home. Receiving a diagnosis was, however, generally regarded as an insult. In 1787 Robert Hamilton (1749–1830) described a case of a soldier suffering from nostalgia, who received sensitive and successful treatment:
- "In the year 1782, while I lay in barracks at Tin mouth in the north of England, a recruit who had lately joined the regiment,...was returned in sick list, with a message from his captain, requesting I would take him into the hospital. He had only been a few months a soldier; was young, handsome, and well-made for the service; but a melancholy hung over his countenance, and wanness preyed on his cheeks. He complained of a universal weakness, but no fixed pain; a noise in his ears, and giddiness of his head....As there were little obvious symptoms of fever, I did not well know what to make of the case...Some weeks passed with little alteration...excepting that he was evidently become more meager. He scarcely took any nourishment...became indolent...He was put on a course of strengthening medicines; wine was allowed him. All proved ineffectual. He had now been in the hospital three months, and was quite emaciated, and like one in the last stage of consumption... On making my morning visit, and inquiring, as usual, of his rest at the nurse, she happened to mention the strong notions he had got in his head, she said, of home, and of his friends. What he was able to speak was constantly on this topic. This I had never heard of before...He had talked in the same style, it seems, less or more, ever since he came into the hospital. I went immediately up to him, and introduced the subject; and from the alacrity with which he resumed it.. I found it a theme which much affected him. He asked me, with earnestness, if I would let him go home. I pointed out to him how unfit he was, from his weakness to undertake such a journey [he was a Welchman] till once he was better; but promised him, assuredly, without farther hesitation, that as soon as he was able he should have six weeks to go home. He revived at the very thought of it... His appetite soon mended; and I saw in less than a week, evident signs of recovery."
In the eighteenth century, scientists were looking for a locus of nostalgia, a nostalgic bone. By the 1850s nostalgia was losing its status as a particular disease and coming to be seen rather as a symptom or stage of a pathological process. It was considered as a form of melancholia and a predisposing condition among suicides. Nostalgia was, however, still diagnosed among soldiers as late as the American Civil War. By the 1870s interest in nostalgia as a medical category had all but vanished. Nostalgia was still being recognized in both the First and Second World Wars, especially by the American armed forces. Great lengths were taken to study and understand the condition to stem the tide of troops leaving the front in droves (see the BBC documentary Century of the Self).
One notable recent medical study has looked at the physiological affects thinking about past 'good' memories can have. They found that thinking about the past 'fondly' actually increased perceptions of warmth, meaning being nostalgic can make you actually feel warmer 
As a description 
Nostalgia is triggered by something reminding an individual of an event or item from their past. The resulting emotion can vary from happiness to sorrow. The term of "feeling nostalgic" is more commonly used to describe pleasurable emotions associated with and/or a longing to go back to a particular period of time, although the former may also be true.
As a deception 
One recent study critiques the idea of nostalgia, which in some forms can become a defense mechanism by which people avoid the historical facts.  This study looked at the different portrayals of apartheid in South Africa and argued that nostalgia appears as two ways, 'restorative nostalgia' a wish to return to that past, and 'reflective nostalgia' which is more critically aware.
Swiss nostalgia was linked to the singing of Kuhreihen, which were forbidden to Swiss mercenaries because they led to nostalgia to the point of desertion, illness or death. The 1767 Dictionnaire de Musique by Jean-Jacques Rousseau claims that Swiss mercenaries were threatened with severe punishment to prevent them from singing their Swiss songs. It became somewhat of a topos in Romantic literature, and figures in the poem Der Schweizer by Achim von Arnim (1805) and in Clemens Brentano's Des Knaben Wunderhorn (1809) as well as in the opera Le Chalet by Adolphe Charles Adam (1834) which was performed for Queen Victoria under the title The Swiss Cottage. The Romantic connection of nostalgia, the Kuhreihen and the Swiss Alps was a significant factor in the enthusiasm for Switzerland, the development of early tourism in Switzerland and Alpinism that took hold of the European cultural elite in the 19th century. German Romanticism coined an opposite to Heimweh, Fernweh "far-sickness," "longing to be far away," like wanderlust expressing the Romantic desire to travel and explore.
See also 
- Golden age (metaphor)
- Historic preservation
- Mono no aware
- The Nostalgia Critic
- Old-time radio
- Retro style
||This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (January 2008)|
- Boym, Svetlana (2002). The Future of Nostalgia. Basic Books. pp. xiii–xiv. ISBN 0-465-00708-2.
- Boym, S. (2001). The future of nostalgia. New York, NY: Basic Books
- Wisconsin Public Radio, To the Best of Our Knowledge, "Svetlana Boym on Nostalgia", 2002 November 3
- Heartwarming memories: Nostalgia maintains physiological comfort. Zhou, Xinyue; Wildschut, Tim; Sedikides, Constantine; Chen, Xiaoxi; Vingerhoets, Ad J. J. M.Emotion, Vol 12(4), Aug 2012, 678-684. doi: 10.1037/a0027236
- Hook, D.(2012) Screened history: Nostalgia as defensive formation Peace and Conflict: Journal of Peace Psychology, Vol 18(3), Aug, 2012. Special issue: Of Narratives and Nostalgia. pp. 225-239
- Simon Bunke: Heimweh. Studien zur Kultur- und Literaturgeschichte einer tödlichen Krankheit. (Homesickness. On the Cultural and Literary History of a Lethal Disease). Freiburg 2009. 674 pp.
- Boulbry, Gaëlle and Borges, Adilson. Évaluation d’une échelle anglo-saxonne de mesure du tempérament nostalgique dans un contexte culturel français (Evaluation of an anglo-saxon scale of measurement of nostalgic mood in a French cultural context)
- Dominic Boyer, "Ostalgie and the Politics of the Future in Eastern Germany." Public Culture 18(2):361-381.
- Simon Bunke: Heimweh. In: Bettina von Jagow / Florian Steger (Eds.): Literatur und Medizin im europäischen Kontext. Ein Lexikon. Göttingen: Vandenhoeck & Ruprecht 2005. Sp. 380-384.
- Coromines i Vigneaux, Joan. Diccionari etimològic i complementari de la llengua catalana [Barcelona, Curial Edicions Catalanes, 1983]
- Davis, Fred Yearning for Yesterday: a Sociology of Nostalgia. New York: Free Press, 1979.
- Hofer, Johannes, "Medical Dissertation on Nostalgia." Bulletin of The Institute of the History of Medicine. Trans. Carolyn Kiser Anspach 2.6 ((1688) Aug. 1934): 376-91.
- Hunter, Richard and Macalpine, Ida. Three Hundred Years of Psychiatry:1535–1860, [Hartsdale, NY, Carlisle Publishing, Inc, 1982]
- Hutcheon, Linda "Irony, Nostalgia, and the Postmodern"
- Jameson, Fredric "Nostalgia for the Present." The South Atlantic Quarterly, 88.2 (1989): 527. 60.
- Thurber, Christopher A. and Marian D. Sigman, "Preliminary Models of Risk and Protective Factors for Childhood Homesickness: Review and Empirical Synthesis." Child Development 69:4 (Aug. 1998): 903-34.
- Dylan Trigg, The Aesthetics of Decay: Nothingness, Nostalgia, and the Absence of Reason (New York: Peter Lang, 2006) 
- Linda M. Austin, 'Emily Brontë's Homesickness', Victorian Studies, 44:4 (summer 2002): 573-596.
- Simon Bunke: Heimwehforschung.de
- BBC Four Documentaries - The Century of the Self