infiltratev. i.[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.()The water infiltrates through the porous rock. (Addison.)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.()v. t. To penetrate gradually; -- sometimes used reflexively.(J. S. Mill.)