Source : Webster's Revised Unabridged Dictionary (1913)

Nostalgia \Nos*tal"gi*a\, n. [NL., fr. Gr. ? a return home + ?
   pain.] (Med.)
   Homesickness; esp., a severe and sometimes fatal form of
   melancholia, due to homesickness.

Source : WordNet®

nostalgia
     n : longing for something past