Source : Webster's Revised Unabridged Dictionary (1913)

Impervious \Im*per"vi*ous\, a. [L. impervius; pref. im- not +
   per through + via way. See {Voyage}.]
   Not pervious; not admitting of entrance or passage through;
   as, a substance impervious to water or air.

         This gulf impassable, impervious.        --Milton.

         The minds of these zealots were absolutely impervious.
                                                  --Macaulay.

   Syn: Impassable; pathless; impenetrable; imperviable;
        impermeable. -- {Im*per"vi*ous*ly}, adv. --
        {Im*per"vi*ous*ness}, n.

Source : WordNet®

imperviousness
     n : resistance to penetration by a liquid [syn: {impenetrability}]
         [ant: {penetrability}, {penetrability}]