1. [n]  any heavy material used to stabilize a ship or airship
2. [v]  make steady with a ballast

BALLAST is a 7 letter word that starts with B.

1. \Bal"last\, n. [D. ballast; akin to Dan. baglast,
   ballast, OSw. barlast, Sw. ballast. The first part is perh.
   the same word as E. bare, adj.; the second is last a burden,
   and hence the meaning a bare, or mere, load. See {Bare}, a.,
   and {Last} load.]
   1. (Naut.) Any heavy substance, as stone, iron, etc., put
      into the hold to sink a vessel in the water to such a
      depth as to prevent capsizing.
   2. Any heavy matter put into the car of a balloon to give it
      steadiness.
   3. Gravel, broken stone, etc., laid in the bed of a railroad
      to make it firm and solid.
   4. The larger solids, as broken stone or gravel, used in
      making concrete.
   5. Fig.: That which gives, or helps to maintain, uprightness,
      steadiness, and security.
            It [piety] is the right ballast of prosperity.
                                                  --Barrow.
   {Ballast engine}, a steam engine used in excavating and for
      digging and raising stones and gravel for ballast.
   {Ship in ballast}, a ship carring only ballast.
   

2. \Bal"last\, v. t. [imp. & p. p. {Ballasted}; p. pr. &
   vb. n. {Ballasting}.]
   1. To steady, as a vessel, by putting heavy substances in the
      hold.
   2. To fill in, as the bed of a railroad, with gravel, stone,
      etc., in order to make it firm and solid.
   3. To keep steady; to steady, morally.
            'T is charity must ballast the heart. --Hammond.