The Collaborative International Dictionary of English v.0.48:

nostalgia \nos*tal"gi*a\, n. [NL., fr. Gr. ? a return home + ?
   pain.]
   1. (Med.) Homesickness; esp., a severe and sometimes fatal
      form of melancholia, due to homesickness.
      [1913 Webster]

   2. A sentimental yearning to return to an earlier time
      remembered as happier or more pleasant, or a former place
      evoking happy memories; a longing to experience again a
      former happy time; as, a nostalgia for the brotherhood of
      the Woodstock music festival; a nostalgia for the
      comradeship of one's college friends.
      [PJC]

WordNet (r) 3.0 (2006):

nostalgia
    n 1: longing for something past