The term nostalgia describes a sentimentality for the past, typically for a period or place with happy personal associations. The word is a learned, meaning homecoming; a Homeric word meaning pain or ache and was coined by a 17th Century medical student to describe 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 usage, nostalgia can refer to a general interest in the past, personalities and events, especially "the good old days" from one's earlier life.
Scientific literature on nostalgia has attempted to pin down 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.