1. n. 
A permeable material used in a filtration device through which filtrate passes and on which the filter cake is deposited, commonly a specifically designed filter paper or permeable disk used in a static filter press that meets API standards. The filter medium can be the cylindrical, permeable core or disk used in a dynamic filtration test or permeable rock downhole on which a filter cake is deposited in a wellbore.
Synonyms: filter media
See: capillary suction time test, dynamic filter press, filter press, fluid-loss control, fluid-loss-control material, high-pressure, high-temperature filtration test, low-pressure, low-temperature filtration test, particle-plugging apparatus, relative filtrate volume, static filtration, wastewater cleanup, water clarification