The Collaborative International Dictionary of English v.0.48:
Perviousness \Per"vi*ous*ness\, n. The quality or state of being pervious; as, the perviousness of glass. --Boyle. [1913 Webster]WordNet (r) 3.0 (2006):
perviousness n 1: the quality of being penetrable (by people or light or missiles etc.) [syn: penetrability, perviousness] [ant: impenetrability, imperviousness]