The Collaborative International Dictionary of English v.0.48:

Interpenetration \In`ter*pen`e*tra"tion\, n.
   The act or process of penetrating between or within other
   substances; mutual penetration; also, the result of a process
   of interpenetration. --Milman.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

interpenetration
    n 1: the action of penetrating between or among
    2: mutual penetration; diffusion of each through the other [syn:
       interpenetration, permeation]