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. (Science: 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.