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.

Source: Webster's Revised Unabridged Dictionary (1913)