To enter by penetrating the pores or interstices of a substance; to filter into or through something.
- 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.
- To gain entrance gradually or surreptitiously.
- One that infiltrates, especially an abnormal substance that accumulates gradually in cells or body tissues.