Source : Webster's Revised Unabridged Dictionary (1913)

Incompressibility \In`com*press`i*bil"i*ty\, n. [Cf. F.
   incompressibilit['e].]
   The quality of being incompressible, or incapable of
   reduction in volume by pressure; -- formerly supposed to be a
   property of liquids.

         The incompressibility of water is not absolute. --Rees.

Source : WordNet®

incompressibility
     n : the property of being incompressible [ant: {compressibility}]