The Collaborative International Dictionary of English v.0.48:

Melodeon \Me*lo"de*on\, n. [NL., fr. Gr. ? musical. See
   Melody, and cf. Odeon.]
   [1913 Webster]
   1. (Mus.) A kind of small reed organ; -- a portable form of
      the seraphine.
      [1913 Webster]

   2. A music hall.
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "melodeon":
   Hammond organ, baroque organ, calliope, calliophone, choralcelo,
   electric organ, electro-pneumatic organ, harmonium,
   hydraulic organ, melodica, orchestrelle, organ, organophone,
   pipe organ, reed organ, seraphine, steam organ, symphonion,
   tracker-action organ, tubular-pneumatic organ, vocalion