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

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "nostalgia":
   Heimweh, aching, bathos, bleeding heart, cloyingness, daydream,
   daydreaming, desiderium, goo, hankering, hearts-and-flowers,
   homesickness, honing, languishing, languishment, longing,
   mal du pays, maladie du pays, maudlinness, mawkishness, mush,
   mushiness, namby-pamby, namby-pambyism, namby-pambyness,
   nostomania, oversentimentalism, oversentimentality, pining,
   romanticism, sentiment, sentimentalism, sentimentality, slop,
   sloppiness, slush, soap opera, sob story, sweetness and light,
   tearjerker, wishful thinking, wishfulness, wistful eye,
   wistfulness, yearnfulness, yearning, yen