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.
      [1913 Webster +PJC]

   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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The Collaborative International Dictionary of English v.0.48:

Barrel \Bar"rel\ (b[a^]r"r[e^]l), n.[OE. barel, F. baril, prob.
   fr. barre bar. Cf. Barricade.]
   1. A round vessel or cask, of greater length than breadth,
      and bulging in the middle, made of staves bound with
      hoops, and having flat ends or heads; as, a cracker
      barrel. Sometimes applied to a similar cylindrical
      container made of metal, usually called a drum.
      [1913 Webster +PJC]

   2. The quantity which constitutes a full barrel. This varies
      for different articles and also in different places for
      the same article, being regulated by custom or by law. A
      barrel of wine is 311/2 gallons; a barrel of flour is 196
      pounds.
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   3. A solid drum, or a hollow cylinder or case; as, the barrel
      of a windlass; the barrel of a watch, within which the
      spring is coiled.
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   4. A metallic tube, as of a gun, from which a projectile is
      discharged. --Knight.
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   5. A jar. [Obs.] --1 Kings xvii. 12.
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   6. (Zool.) The hollow basal part of a feather.
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   Barrel bulk (Com.), a measure equal to five cubic feet,
      used in estimating capacity, as of a vessel for freight.


   Barrel drain (Arch.), a drain in the form of a cylindrical
      tube.

   Barrel of a boiler, the cylindrical part of a boiler,
      containing the flues.

   Barrel of the ear (Anat.), the tympanum, or tympanic
      cavity.

   Barrel organ, an instrument for producing music by the
      action of a revolving cylinder.

   Barrel vault. See under Vault.
      [1913 Webster]

WordNet (r) 3.0 (2006):

barrel organ
    n 1: a musical instrument that makes music by rotation of a
         cylinder studded with pegs [syn: barrel organ, grind
         organ, hand organ, hurdy gurdy, hurdy-gurdy, street
         organ]