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A filter is a membrane or layer that is designed to block certain things (objects or substances) whilst letting others through. Filters are often used to remove harmful substances from air or water, for example to reduce air pollution or to make contaminated air breathable with a gas mask. Very common is the coffee filter.
In mathematics, a filter is a subset of power set with certain properties. See filter (mathematics).
A filter is a short-form of mail filter.
An electronic filter is an electrical circuit whose frequency response or transfer function is non-uniform. In other words, its gain or attenuation depends on frequency.
An analog filter is a form of filter (usually electronic) that uses analog circuitry in its implementation.
A digital filter is an algorithm for processing numeric data, and which can also be used in digital signal processors to function in a similar way to an electronic filter. Digital filters are now used in many modern appliances, such as CD players, radio tuners, TVs etc.
An audio filter is a type of filter that is used for processing audio signals in appliances such as CD players, synthesizers or sound effects.
A 1990s industrial rock band is called Filter.
An optical filter removes/only passes certain wavelengths of light, e. g. Infrared filter, ND filter, UV filter

See also: Glossary of filter terms\n