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.
      [1913 Webster]

   3. Cheerfulness; vivacity; liveliness; sprightliness; -- the
      opposite of heaviness; as, buoyancy of spirits.
      [1913 Webster]

WordNet (r) 3.0 (2006):

buoyancy
    n 1: cheerfulness that bubbles to the surface [syn: buoyancy,
         perkiness]
    2: the property of something weightless and insubstantial [syn:
       airiness, buoyancy]
    3: the tendency to float in water or other liquid
    4: irrepressible liveliness and good spirit; "I admired his
       buoyancy and persistent good humor" [syn: irrepressibility,
       buoyancy]