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]

Moby Thesaurus II by Grady Ward, 1.0:

140 Moby Thesaurus words for "infiltrate":
   absorb, adsorb, advance against, advance upon, assimilate,
   barge in, bathe, bear down upon, besprinkle, blot, blot up,
   break in, break in upon, breathe, brew, burst in, butt in,
   charge in, chemisorb, chemosorb, color, come between,
   counterattack, crash, crash in, crash the gates, creep in,
   crowd in, cut in, decoct, digest, douche, dredge, drench, drink,
   drink in, drink up, drive, drouk, dye, edge in, elbow in, encroach,
   engross, entincture, entrench, filter in, flank, flavor, flush,
   foist, foist in, gas, horn in, imbibe, imbrue, imbue, impinge,
   impose, impose on, impose upon, impregnate, infringe, infuse,
   inject, insinuate, instill, interfere, interlope, interpose,
   intervene, intrude, invade, irrupt, launch an attack, lave, leach,
   leak in, leaven, lixiviate, macerate, march against, march upon,
   mount an attack, obtrude, open an offensive, osmose, penetrate,
   percolate, percolate in, percolate into, permeate, pervade,
   press in, push, push in, put on, put upon, rinse, rush in,
   saturate, season, seep in, seethe, slink in, slip in, slurp up,
   smash in, sneak in, soak, soak in, soak up, sodden, sop, sorb,
   souse, sponge, squeeze in, steal in, steep, storm in, strike,
   suffuse, swill up, take in, take up, temper, throng in, thrust,
   thrust in, tincture, tinge, transfuse, trench, trespass, wash,
   waterlog, work in, worm, worm in