Technically, the term nostalgia describes a particular sentimentality towards a former experience. In simpler terms, it is a positive reflection of the past, and a powerful one at that.
Nostalgia is generally triggered by some sort of cue from the past. This sentimentality is then associated with a period or place with happy personal memories. The term itself was coined by a 17th-century medical student as a way to describe the anxieties displayed by Swiss mercenaries fighting away from home.
In later years, as science has learned more about the phenomena, nostalgia has been widely considered an independent and even positive emotion that people experience often.