The Collaborative International Dictionary of English v.0.48:

Harmonica \Har*mon"i*ca\ (-[i^]*k[.a]), n. [Fem. fr. L.
   harmonicus harmonic. See Harmonic, n. ]
   1. A musical instrument, consisting of a series of
      hemispherical glasses which, by touching the edges with
      the dampened finger, give forth the tones; it is now
      called the glass harmonica, to distinguish it from the
      common harmonica, formerly called the harmonicon.
      [1913 Webster]

   2. A toy instrument of strips of glass or metal hung on two
      tapes, and struck with hammers.
      [1913 Webster]

   3. A small wind musical instrument shaped like a flat bar
      with holes along the thin edges, held in the hand and
      producing notes from multiple vibrating reeds arranged
      inside along its length; it was formerly called the
      harmonicon. See harmonicon.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Harmonicon \Har*mon"i*con\ (-[i^]*k[o^]n), n.
   A small, flat, wind instrument of music, in which the notes
   are produced by the vibration of free metallic reeds; it is
   now called the harmonica.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

harmonica
    n 1: a small rectangular free-reed instrument having a row of
         free reeds set back in air holes and played by blowing into
         the desired hole [syn: harmonica, mouth organ, harp,
         mouth harp]