The word nostalgia comes from a combination of two Greek words, νόστος (nóstos), meaning "homecoming," and ἄλγος (álgos), meaning "pain, ache."
It is attributed to a 17th-century medical student to describe anxieties displayed by Swiss mercenaries fighting away from home.
It can be brought on by many different associations. Memories can be stirred when looking at old photos of people and places, listening to a song that takes you back to when you first heard it, or tasting something familiar from your childhood.
Sensory expert Professor Barry Smith says that "Smell, more than any other sense, can evoke powerful, emotional memories. Whole scenes of people, places and things can be brought back to life by the hint of a long forgotten scent."