The Collaborative International Dictionary of English v.0.48:

Buoyancy \Buoy"an*cy\, n.; pl. Buoyancies.
   1. The property of floating on the surface of a liquid, or in
      a fluid, as in the atmosphere; specific lightness, which
      is inversely as the weight compared with that of an equal
      volume of water.
      [1913 Webster]

   2. (Physics) The upward pressure exerted upon a floating body
      by a fluid, which is equal to the weight of the body;
      hence, also, the weight of a floating body, as measured by
      the volume of fluid displaced.
      [1913 Webster]

            Such are buoyancies or displacements of the
            different classes of her majesty's ships. --Eng.
                                                  Cyc.
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   3. Cheerfulness; vivacity; liveliness; sprightliness; -- the
      opposite of heaviness; as, buoyancy of spirits.
      [1913 Webster]