Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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.

            Such are buoyancies or displacements of the
            different classes of her majesty's ships. --Eng.
                                                  Cyc.

   3. Cheerfulness; vivacity; liveliness; sprightliness; -- the
      opposite of {heaviness}; as, buoyancy of spirits.