means A wistful, sentimental longing for the past — for a time, place, or version of life now gone.

from Coined in the 17th century by a Swiss medical student, Johannes Hofer, who stitched together two Greek roots — nostos, 'homecoming,' and algos, 'pain' — to name what he saw as a genuine disease: the desperate, sometimes fatal homesickness of Swiss mercenaries stranded far from their mountains. For a long time nostalgia was a diagnosis, not a mood; only later did it soften from a soldier's ache for home into our general tender ache for the past.