The Collaborative International Dictionary of English v.0.48:

Ballast \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.
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   2. To fill in, as the bed of a railroad, with gravel, stone,
      etc., in order to make it firm and solid.
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   3. To keep steady; to steady, morally.
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            'T is charity must ballast the heart. --Hammond.
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The Collaborative International Dictionary of English v.0.48:

Ballast \Bal"last\ (b[a^]l"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.
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   2. Any heavy matter put into the car of a balloon to give it
      steadiness.
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   3. Gravel, broken stone, etc., laid in the bed of a railroad
      to make it firm and solid.
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   4. The larger solids, as broken stone or gravel, used in
      making concrete.
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   5. Fig.: That which gives, or helps to maintain, uprightness,
      steadiness, and security.
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            It [piety] is the right ballast of prosperity.
                                                  --Barrow.
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   Ballast engine, a steam engine used in excavating and for
      digging and raising stones and gravel for ballast.

   Ship in ballast, a ship carrying only ballast.
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WordNet (r) 3.0 (2006):

ballast
    n 1: any heavy material used to stabilize a ship or airship
    2: coarse gravel laid to form a bed for streets and railroads
    3: an attribute that tends to give stability in character and
       morals; something that steadies the mind or feelings
    4: a resistor inserted into a circuit to compensate for changes
       (as those arising from temperature fluctuations) [syn:
       ballast resistor, ballast, barretter]
    5: an electrical device for starting and regulating fluorescent
       and discharge lamps [syn: ballast, light ballast]
    v 1: make steady with a ballast