The term nostalgia describes a yearing for the past, often in idealized form. The word is a learned formation of Greek compounds, consisting of "returing home," a Homeric word, and "pain" or "ache." It was described as a medical condition, a form of melancholy, in the Early Modem period, and came to be an important topic in Romanticism. In common, less clinical usage, nostalgia sometimes includes a general interest in past eras and their personalities and events, especially the good old days of a few generations back recast in an idyllic light.
Sometimes it is brought on by a sudden image, or remembrance of something from one's childhood.