that portion of a filtration system that provides the liquid-solid separation, such as close-woven textiles or metal screens, papers, nonwoven fabrics, granular beds, or porous media, 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.