1. [n]  an outlying farm building for storing grain or animal feed and housing farm animals
2. [n]  (atomic or nuclear physics) a unit of nuclear cross section; the effective circular area that one particle presents to another as a target for an encounter

BARN is a 4 letter word that starts with B.

1. \Barn\, n. [OE. bern, AS. berern, bern; bere barley + ern,
   [ae]rn, a close place. ?92. See {Barley}.]
   A covered building used chiefly for storing grain, hay, and
   other productions of a farm. In the United States a part of
   the barn is often used for stables.
   
   {Barn owl} (Zo["o]l.), an owl of Europe and America ({Aluco
      flammeus}, or {Strix flammea}), which frequents barns and
      other buildings.
   
   {Barn swallow} (Zo["o]l.), the common American swallow
      ({Hirundo horreorum}), which attaches its nest of mud to
      the beams and rafters of barns.
   
   

2. \Barn\, v. t.
   To lay up in a barn. [Obs.] --Shak.
   
         Men . . . often barn up the chaff, and burn up the
         grain.                                   --Fuller.
   
   

3. \Barn\, n.
   A child. [Obs.] See {Bairn}.
   
   

a storehouse (Deut. 28:8; Job 39:12; Hag. 2:19) for grain, which was usually under ground, although also sometimes above ground (Luke 12:18).