ˈɪnfɪltɹeɪt ˈɪnfəltɹeɪt ˈpɛnɪtɹeɪt pass through an enemy line; in a military conflict pass into or through by filtering or permeating the substance infiltrated the material cause (a liquid) to enter by penetrating the interstices enter a group or organization in order to spy on the members The student organization was infiltrated by a traitor