To pass (troops, for example) surreptitiously into enemy-held territory. To penetrate with hostile intent: infiltrate enemy lines; terrorists that had infiltrated the country. To enter or take up positions in gradually or surreptitiously, as for purposes of espionage or takeover: infiltrated key government agencies with spies. To cause (a liquid, for example) to permeate a substance by passing through its interstices or pores. To permeate (a porous substance) with a liquid or gas.