The Collaborative International Dictionary of English v.0.48:

Infiltrate \In*fil"trate\, v. i. [imp. & p. p. Infiltrated; p.
   pr. & vb. n. Infiltrating.] [Pref. in- + filtrate: cf. F,
   s'infiltrer. Cf. Infilter.]
   1. To enter by penetrating the pores or interstices of a
      substance; to filter into or through something.
      [1913 Webster]

            The water infiltrates through the porous rock.
                                                  --Addison.
      [1913 Webster]

   2. To enter secretly (into an organization, territory, etc.)
      for hostile purposes, such as spying or sabotage; as, the
      FBI infiltrated into the U. S. communist party's upper
      echelons; also used transitively; as, to infiltrate the
      opponent's organization; to infiltrate one's agents into a
      hostile country.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Infiltrate \In*fil"trate\, v. t.
   To penetrate gradually; -- sometimes used reflexively. --J.
   S. Mill.
   [1913 Webster]

WordNet (r) 3.0 (2006):

infiltrate
    v 1: cause (a liquid) to enter by penetrating the interstices
    2: enter a group or organization in order to spy on the members;
       "The student organization was infiltrated by a traitor" [syn:
       infiltrate, penetrate]
    3: pass into or through by filtering or permeating; "the
       substance infiltrated the material"
    4: pass through an enemy line; in a military conflict [syn:
       infiltrate, pass through]