The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

         The incompressibility of water is not absolute. --Rees.
   [1913 Webster]

WordNet (r) 3.0 (2006):

incompressibility
    n 1: the property of being incompressible [ant:
         compressibility, sponginess, squeezability]