1. A person or something that can puncture or pierce: Floyd had penetrants that were powerful arguments against the policies of the current company president.
2. A compound that goes through or into something: The pharmacist recommended a lotion for Leon's skin that could function as a penetrant to relieve the itching.
3. A large capsule that discharges and injects a toxic fluid: The jellyfish uses a penetrant containing a barbed, threadlike tube, that delivers a paralyzing sting into its attackers or prey.

A reference to someone or something which is sharp and that has the power to break in or to force its way into a place or a situation: The penetrant cold weather was difficult to live in because of the penetrant wind from the north.

A penetrant liquid is one that usually contains a dye or fluorescent chemical which, when poured over a metal surface, determines the existence of cracks.