Vapor Pressure
The pressure exerted when a solid or liquid is in equilibrium with its own vapor.  The higher the vapor pressures the higher the volatility.
Volatility
The ability of a substance to change from liquid or solid form to a gaseous form.
Volume of distribution (VD)
The volume of body fluid in which a compound is apparently distributed.  It may consist of plasma, interstitial fluid, and intercellular fluid.