The Collaborative International Dictionary of English v.0.48:

Organ \Or"gan\ ([^o]r"gan), n. [L. organum, Gr. 'o`rganon; akin
   to 'e`rgon work, and E. work: cf. F. organe. See Work, and
   cf. Orgue, Orgy.]
   1. An instrument or medium by which some important action is
      performed, or an important end accomplished; as,
      legislatures, courts, armies, taxgatherers, etc., are
      organs of government.
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   2. (Biol.) A natural part or structure in an animal or a
      plant, capable of performing some special action (termed
      its function), which is essential to the life or
      well-being of the whole; as, the heart, lungs, etc., are
      organs of animals; the root, stem, foliage, etc., are
      organs of plants.
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   Note: In animals the organs are generally made up of several
         tissues, one of which usually predominates, and
         determines the principal function of the organ. Groups
         of organs constitute a system. See System.
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   3. A component part performing an essential office in the
      working of any complex machine; as, the cylinder, valves,
      crank, etc., are organs of the steam engine.
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   4. A medium of communication between one person or body and
      another; as, the secretary of state is the organ of
      communication between the government and a foreign power;
      a newspaper is the organ of its editor, or of a party,
      sect, etc. A newsletter distributed within an organization
      is often called its house organ.
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   5. [Cf. AS. organ, fr. L. organum.] (Mus.) A wind instrument
      containing numerous pipes of various dimensions and kinds,
      which are filled with wind from a bellows, and played upon
      by means of keys similar to those of a piano, and
      sometimes by foot keys or pedals; -- formerly used in the
      plural, each pipe being considered an organ.
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            The deep, majestic, solemn organs blow. --Pope.
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   Note: Chaucer used the form orgon as a plural.
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               The merry orgon . . . that in the church goon
               [go].
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   Barrel organ, Choir organ, Great organ, etc. See under
      Barrel, Choir, etc.

   Cabinet organ (Mus.), an organ of small size, as for a
      chapel or for domestic use; a reed organ.

   Organ bird (Zool.), a Tasmanian crow shrike (Gymnorhina
      organicum). It utters discordant notes like those of a
      hand organ out of tune.

   Organ fish (Zool.), the drumfish.

   Organ gun. (Mil.) Same as Orgue
      (b) .

   Organ harmonium (Mus.), an harmonium of large capacity and
      power.

   Organ of Corti (Anat.), a complicated structure in the
      cochlea of the ear, including the auditory hair cells, the
      rods or fibers of Corti, the membrane of Corti, etc. See
      Note under Ear.

   Organ pipe. See Pipe, n., 1.

   Organ-pipe coral. (Zool.) See Tubipora.

   Organ point (Mus.), a passage in which the tonic or
      dominant is sustained continuously by one part, while the
      other parts move.
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WordNet (r) 3.0 (2006):

organ of Corti
    n 1: the hearing organ of the inner ear; contains receptors that
         respond to sound waves