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.
      [1913 Webster]

   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.
      [1913 Webster]

   4. A metallic tube, as of a gun, from which a projectile is
      discharged. --Knight.
      [1913 Webster]

   5. A jar. [Obs.] --1 Kings xvii. 12.
      [1913 Webster]

   6. (Zool.) The hollow basal part of a feather.
      [1913 Webster]

   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]

The Collaborative International Dictionary of English v.0.48:

Barrel \Bar"rel\ (b[a^]r"r[e^]l), v. t. [imp. & p. p. Barreled
   (-r[e^]ld), or Barrelled; p. pr. & vb. n. Barreling, or
   Barrelling.]
   To put or to pack in a barrel or barrels.
   [1913 Webster]

WordNet (r) 3.0 (2006):

barrel
    n 1: a tube through which a bullet travels when a gun is fired
         [syn: barrel, gun barrel]
    2: a cylindrical container that holds liquids [syn: barrel,
       cask]
    3: a bulging cylindrical shape; hollow with flat ends [syn:
       barrel, drum]
    4: the quantity that a barrel (of any size) will hold [syn:
       barrel, barrelful]
    5: any of various units of capacity; "a barrel of beer is 31
       gallons and a barrel of oil is 42 gallons" [syn: barrel,
       bbl]
    v 1: put in barrels