The term nostalgia describes a sentimental longing 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". It was described as a medical condition, a form of melancholy, in the Early Modern period, and became an important trope in Romanticism.
Nostalgia, in its most common form, was responsible for the old front desk of The Beverly Hills Hotel (from 1942 to 1979) being made into a bar.
In common, less clinical usage, nostalgia sometimes includes a general interest in past eras and their personalities and events, especially the "good old days," such as a sudden image, or remembrance of something from one's childhood.
example: I felt a touch of nostalgia watching the Olympics tonight, I remembered the good old days when Yelena was still a woman.