Source : Webster's Revised Unabridged Dictionary (1913)

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

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

   6. (Zo["o]l.) The hollow basal part of a feather.

   {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}.

Source : WordNet®

barrel vault
     n : the simplest form of vault; a single continuous arch