The Collaborative International Dictionary of English v.0.48:

Vault \Vault\ (v[add]lt; see Note, below), n. [OE. voute, OF.
   voute, volte, F. vo[^u]te, LL. volta, for voluta, volutio,
   fr. L. volvere, volutum, to roll, to turn about. See
   Voluble, and cf. Vault a leap, Volt a turn, Volute.]
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   1. (Arch.) An arched structure of masonry, forming a ceiling
      or canopy.
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            The long-drawn aisle and fretted vault. --Gray.
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   2. An arched apartment; especially, a subterranean room, used
      for storing articles, for a prison, for interment, or the
      like; a cell; a cellar. "Charnel vaults." --Milton.
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            The silent vaults of death.           --Sandys.
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            To banish rats that haunt our vault.  --Swift.
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   3. The canopy of heaven; the sky.
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            That heaven's vault should crack.     --Shak.
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   4. [F. volte, It. volta, originally, a turn, and the same
      word as volta an arch. See the Etymology above.] A leap or
      bound. Specifically:
      (a) (Man.) The bound or leap of a horse; a curvet.
      (b) A leap by aid of the hands, or of a pole, springboard,
          or the like.
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   Note: The l in this word was formerly often suppressed in
         pronunciation.
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   Barrel vault, Cradle vault, Cylindrical vault, or
   Wagon vault (Arch.), a kind of vault having two parallel
      abutments, and the same section or profile at all points.
      It may be rampant, as over a staircase (see Rampant
      vault, under Rampant), or curved in plan, as around the
      apse of a church.

   Coved vault. (Arch.) See under 1st Cove, v. t.

   Groined vault (Arch.), a vault having groins, that is, one
      in which different cylindrical surfaces intersect one
      another, as distinguished from a barrel, or wagon, vault.


   Rampant vault. (Arch.) See under Rampant.

   Ribbed vault (Arch.), a vault differing from others in
      having solid ribs which bear the weight of the vaulted
      surface. True Gothic vaults are of this character.

   Vault light, a partly glazed plate inserted in a pavement
      or ceiling to admit light to a vault below.
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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.
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   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.
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WordNet (r) 3.0 (2006):

barrel vault
    n 1: the simplest form of vault; a single continuous arch