The Collaborative International Dictionary of English v.0.48:

Interpenetrate \In`ter*pen"e*trate\, v. i.
   To penetrate each the other; to penetrate between bodies or
   their parts.
   [1913 Webster]

   Interpenetrating molding (Arch.), in late Gothic
      architecture, a decoration by means of moldings which seem
      to pass through solid uprights, transoms, or other
      members; often, two sets of architectural members
      penetrating one another, in appearance, as if both had
      been plastic when they were put together.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Interpenetrate \In`ter*pen"e*trate\, v. t.
   To penetrate between or within; to penetrate mutually.
   [1913 Webster]

         It interpenetrates my granite mass.      --Shelley.
   [1913 Webster]

WordNet (r) 3.0 (2006):

interpenetrate
    v 1: penetrate mutually or be interlocked; "The territories of
         two married people interpenetrate a lot" [syn:
         interpenetrate, permeate]
    2: spread or diffuse through; "An atmosphere of distrust has
       permeated this administration"; "music penetrated the entire
       building"; "His campaign was riddled with accusations and
       personal attacks" [syn: permeate, pervade, penetrate,
       interpenetrate, diffuse, imbue, riddle]