Filtration is the separation of a fluid-solids mixture, involving passage of most of the liquid through a porous barrier that retains most of the solids contained in the mixture. Filtration is the term for the unit operation. A filter is a piece of equipment by which filtration is performed. The filter medium is the barrier that lets the liquid pass, while retaining a large part of the solids. It may be a screen, cloth, paper, or a bed of solids. The liquid that passes through the filter medium is called the filtrate.