Nostalgia (n.) primarily refers to a sentimental longing for things, persons or situations that are familiar and are not present. Subsequently, it can be defined as clinical homesickness. This definition of the word is especially regarded as a medical condition. Secondly, in the more common and less clinical usage, the term is used as a description of a general interest in past eras and their personalities and events. This denotes “the good old days” or a time in one’s former life with a sudden image or remembrance of something from one’s past. Nostalgia involves a wistful and sentimental yearning for something, someone or some situation which for some reason is unattainable at that specific period in one’s life. It can be a longing for or regretful memory or period of the past, that is especially in one’s own lifetime.  Thus, it must originate from a past period or irrecoverable condition. 
In regard to the more medical sense of the term, it was coined in 1688 by a medical student named Johannes Hofer in is Basel dissertation. When he introduced nostalgia, he associated it to the condition also known as mal du Suisse “Swiss illness” or schweizerheimweh “Swiss Homesickness”. It was formed because of its consistent occurrence in the lives of Swiss mercenaries as they were in the plains and lowlands of France or Italy and yearned for their native mountain landscapes.  The term was then created as a New Latin rendering of German heimweh, from Greek algos pertaining to “pain, grief and distress” and nostos meaning “homecoming” from PIE *nes- which is “to return safely home”.  The transferred sense or main, modern one, of “wistful yearning for the past” was first recorded in 1920. 
Nostalgia is triggered by an emotional reminder of an event or situation from the past. The event is not necessarily happy; the event can vary between its involvement of happiness and sadness. Yet, the latter less commonly evokes a nostalgic feeling. This feeling is more commonly used to describe emotions that are pleasurable, which suggests one’s longing to go back to that period of time. There is a scientific essence to the causes of the feeling, relating heavily to the effect of the five senses of the human body. Smell and touch prove to be the strongest evokers of general memories due to the processing of these stimuli first having to pass through the amygdala or the emotional seat of the brain.  The amygdala influences the retention of emotional memories and hints at the idea of nostalgia being deeply connected to the idea of the general inability to adapt oneself to new surroundings and situations, with the insistence on continually reliving the past. 
Surprisingly, nostalgia may depend precisely on the irrecoverable nature of the past for its emotional impact and appeal. In the twentieth century, as the term became more a psychological condition than a physical one, it became psychically internalized. Thus, from being a curable physical illness, it turned into an incurable one. It was no longer a simple yearning to return home. Immanuel Kant noticed in 1798 that people who returned home were usually slightly disappointed because they in fact didn’t want to return to a place, but rather to a specific time, most likely a time of youth.  Strangely, nostalgia is therefore less about the importance of the past and more about the inability to handle the present.
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