Filtration is the separation of solid particles from a liquid or gas by passing it through a porous mesh on which the solid collects. Commonly used are filter cloths, filter paper, wire mesh, sintered glass, or – where a large volume of water is to be filtered – sand beds. Filters have many varied uses, including use in vacuum cleaners, gasoline and diesel engines, coffee-making, air-conditioning units, water-purification systems, and chemical preparation and analysis.

A magnetic filter is used to remove iron or steel particles from oils, etc., in machine tools and some engines.