Filtration is a process for separating or cleaning substances, usually a suspension or aerosol, and is a mechanical separation process which relies purely on physical phenomena. A filtered liquid is called a filtrate. Loss of pressure is the fall in pressure which occurs during filtration, and it is also called filter resistance.
During surface filtration, the solid particles which are filtered off form a layer on the filter medium, the "filter cake" as it is known. For filtered gases, there is no agreed term, but it is often referred to as "clean gas". The terms "retentate" (restrained phase) and "permeate" (the phase of passing through the filter) are common in membrane technology. The word (originating from Italian filtrare, "to percolate") originally meant "to pass through felt" (German: Filz). This represented the finest filtration method used before the invention of paper, and still today is state-of-the-art in some fields.