Source : Webster's Revised Unabridged Dictionary (1913)

Interpenetration \In`ter*pen`e*tra"tion\, n.
   The act of penetrating between or within other substances;
   mutual penetration. --Milman.

Source : WordNet®

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