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

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "ballast":
   balance, break bulk, break out ballast, clear for action,
   clear the decks, consideration, counterbalance, counterpoise,
   counterweight, equipoise, equivalent, firm, firm up, freeze,
   give-and-take, hold, immobilize, keep, lead, makeweight, nail down,
   offset, pin down, poise, quid pro quo, retain, sandbag, setoff,
   shift ballast, shoot ballast, something of value, stabilitate,
   stabilize, steady, stick, tit for tat, transfix, trim, trim ship,
   trim up, weigh down, weight, weight down, wing out ballast