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

Moby Thesaurus II by Grady Ward, 1.0:

136 Moby Thesaurus words for "barrel":
   abundance, acres, bag, bags, ball the jack, barb, barbule, barrels,
   basket, beeline, bole, boom, bottle, bowl along, box, box up,
   breeze, breeze along, brush, burden, bushel, butt, can, capsule,
   carton, case, cask, cilium, clip, column, copiousness,
   countlessness, crate, cut along, cylinder, cylindroid, drum,
   encase, encyst, filament, filamentule, fill, fleet, flit, flood,
   fly, fly low, foot, freight, go fast, great deal, hamper, hasten,
   heap, heap up, highball, hogshead, hustle, jar, keg, lade,
   lashings, load, lot, lump, make knots, mass, mess, mountain, much,
   multitude, nip, numerousness, ocean, oceans, outstrip the wind,
   pack, pack away, package, parcel, peck, pile, pillar, pipe,
   plenitude, plenty, pocket, pot, pour it on, power, profusion,
   quantities, quantity, quill, rip, rocket, roll, roller, rouleau,
   run, rush, sack, scorch, sea, shaft, ship, sight, sizzle, skim,
   smoke, spate, speed, stack, store, storm along, stow,
   superabundance, superfluity, sweep, tank, tear, tear along,
   thunder along, tin, tons, trunk, tube, tun, volume, whisk, whiz,
   world, worlds, zing, zip, zoom



Easton's 1897 Bible Dictionary:

Barrel
   a vessel used for keeping flour (1 Kings 17:12, 14, 16). The
   same word (cad) so rendered is also translated "pitcher," a
   vessel for carrying water (Gen. 24:14; Judg. 7:16).


Bouvier's Law Dictionary, Revised 6th Ed (1856):

BARREL. A measure of capacity, equal to thirty-six gallons.