The Collaborative International Dictionary of English v.0.48:

Caisson \Cais"son\, n. [F., fr. caisse, case, chest. See 1st
   Case.]
   1. (Mil.)
      (a) A chest to hold ammunition.
      (b) A four-wheeled carriage for conveying ammunition,
          consisting of two parts, a body and a limber. In light
          field batteries there is one caisson to each piece,
          having two ammunition boxes on the body, and one on
          the limber. --Farrow.
      (c) A chest filled with explosive materials, to be laid in
          the way of an enemy and exploded on his approach.
          [1913 Webster]

   2.
      (a) A water-tight box, of timber or iron within which work
          is carried on in building foundations or structures
          below the water level.
      (b) A hollow floating box, usually of iron, which serves
          to close the entrances of docks and basins.
      (c) A structure, usually with an air chamber, placed
          beneath a vessel to lift or float it.
          [1913 Webster]

   3. (Arch.) A sunk panel of ceilings or soffits.
      [1913 Webster]

   Pneumatic caisson (Engin.), a caisson, closed at the top
      but open at the bottom, and resting upon the ground under
      water. The pressure of air forced into the caisson keeps
      the water out. Men and materials are admitted to the
      interior through an air lock. See Lock.
      [1913 Webster]

WordNet (r) 3.0 (2006):

caisson
    n 1: an ornamental sunken panel in a ceiling or dome [syn:
         coffer, caisson, lacuna]
    2: a two-wheeled military vehicle carrying artillery ammunition
    3: a chest to hold ammunition [syn: caisson, ammunition
       chest]
    4: large watertight chamber used for construction under water
       [syn: caisson, pneumatic caisson, cofferdam]