The Collaborative International Dictionary of English v.0.48:

Infiltration \In`fil*tra"tion\, n. [Cf. F. infiltration.]
   [1913 Webster]
   1. The act or process of infiltrating, as of water into a
      porous substance, of a fluid into the cells of an organ or
      part of the body, or of persons into a hostile
      organization.
      [1913 Webster +PJC]

   2. The substance which has entered the pores or cavities of a
      body. --Addison.
      [1913 Webster]

            Calcareous infiltrations filling the cavities.
                                                  --Kirwan.
      [1913 Webster]

   Fatty infiltration. (Med.) See under Fatty.

   Infiltration gallery, a filter gallery.
      [1913 Webster]

WordNet (r) 3.0 (2006):

infiltration
    n 1: a process in which individuals (or small groups) penetrate
         an area (especially the military penetration of enemy
         positions without detection)
    2: the slow passage of a liquid through a filtering medium; "the
       percolation of rainwater through the soil"; "the infiltration
       of seawater through the lava" [syn: percolation,
       infiltration]

Moby Thesaurus II by Grady Ward, 1.0:

159 Moby Thesaurus words for "infiltration":
   absorbency, absorbent, absorption, access, admission, adsorbent,
   adsorption, aerial tactics, aggravated assault, aggression,
   airborne tactics, amphibious attack, applied tactics,
   armed assault, armored tactics, assailing, assailment, assault,
   assimilation, attack, banzai attack, barrier tactics, blitz,
   blitzkrieg, blotter, blotting, blotting paper, breakthrough,
   brewing, cavalry tactics, charge, chemisorption, chemosorption,
   counterattack, counteroffensive, coup de main, crippling attack,
   dead set at, decoction, defensive strategy, descent on, digestion,
   diversion, diversionary attack, drench, drenching, drive, ducking,
   dunking, encirclement, encroachment, endosmosis, engrossment,
   entrance, entree, entrenchment, entry, exosmosis, feint,
   fire tactics, flank attack, frontal attack, gas attack,
   head-on attack, imbruement, imbuement, impingement, import,
   importation, importing, imposition, impregnation, income, incoming,
   incursion, infantry tactics, influx, infringement, infusion,
   ingoing, ingress, ingression, injection, input, inroad, insertion,
   insinuation, instillation, instillment, intake, interference,
   interjection, interloping, interpenetration, interposition,
   interposure, interruption, intervention, introduction,
   introgression, intrusion, invasion, investment, irruption,
   leaching, leakage, lightning attack, lightning war, lixiviation,
   maceration, marination, mass attack, megadeath, mob tactics,
   mobile tactics, mugging, obtrusion, offense, offensive, onset,
   onslaught, osmosis, overkill, panzer warfare, penetration,
   percolation, permeation, pervasion, pincers movement, pulping,
   push, reception, run against, run at, rush, sally, saturation,
   seepage, seething, shock tactics, soak, soakage, soaking, sopping,
   sorption, sortie, souse, sousing, sponge, sponging, steeping,
   strategy, strike, suffusion, tactics, trespass, trespassing,
   unlawful entry, unprovoked assault