# Filter

Filtration is a physical process that separates solid matter and fluid from a mixture.
Filter, filtering, filters or filtration may also refer to:

## Science and technology
### Computing

* Filter (higher-order function), in functional programming
* Filter (software), a computer program to process a data stream
* Filter (video), a software component that performs some operation on a multimedia stream
* Email filtering, the processing of email to organize it according to specified criteria
* Content-control software also known as an Internet filter
* Wordfilter, a script typically used on Internet forums or chat rooms
* Berkeley Packet Filter, filter expression used in the qualification of network data
* DSL filter, a low-pass filter installed between analog devices and a telephone line
* Helicon Filter, a raster graphics editor
* Filter (large eddy simulation), a mathematical operation intended to remove a range of small scales from the solution to the Navier-Stokes equations
* Kalman filter, an approximating algorithm in optimal control applications and problems
* Filter (social media), an appearance-altering digital image effect for social media

### Device

* Filter (chemistry), a device which separates solids from fluids (liquids or gases) by adding a medium through which only the fluid can pass
** Filter (aquarium), critical components of both freshwater and marine aquaria
** Filter paper, a semi-permeable paper barrier placed perpendicular to a liquid or air flow. It is used to separate fine solids from liquids or air
** Air filter, a device composed of fibrous materials which removes solid particulates such as dust, pollen, mold, and bacteria from the air
** Oil filter, a filter to remove contaminants from engine oil, transmission oil, lubricating oil, or hydraulic oil
** Pneumatic filter, a device which removes contaminants from a compressed air stream
** Water filter, removes impurities from water by means of a fine physical barrier, a chemical process or a biological process
** Cigarette filter, a part of a cigarette intended to filter the smoke inhaled by a smoker
** Coffee filter, a utensil used to separate coffee grounds from liquid coffee
** Fuel filter, found in most internal combustion engines
** Filtration (wine)
* Sieve or macroscopic filter, a device for separating wanted elements from unwanted material

### Mathematics

* Filter (mathematics), a special subset of a partially ordered set.
* Filter (set theory), a special family of subsets that forms an (order theoretic) filter with respect to set inclusion
* Filters in topology, the use of collections of subsets to describe convergence.
* Filtering problem (stochastic processes), a mathematical model for a number of filtering problems in signal processing and the like.
* Filtration (mathematics), an indexed set of subobjects of a given algebraic structure S.

### Optics

* Optical filter, selectively transmits light of different wavelengths
** Interference filter, reflects one or more spectral bands or lines and transmits others, while maintaining a nearly zero coefficient of absorption for all wavelengths of interest
** Dichroic filter, a very accurate color filter used to selectively pass light of a small range of colors while reflecting other colors
** Hydrogen-alpha filter, an optical filter that transmits a narrow bandwidth of light centred on the H-alpha wavelength
** Photographic filter, a camera accessory consisting of an optical filter that can be inserted in the optical path
** Infrared cut-off filter, designed to reflect or block mid-infrared wavelengths while passing visible light
** Chelsea filter, a dichromatic optical filter used for identifying coloured stones
** Astronomical filter, a telescope accessory used to enhance the details of celestial objects

### Signal processing

* Filter (signal processing)
** Electronic filter, an electronic circuit which processes signals, for example to remove unwanted frequency components
** Digital filter, a system that performs mathematical operations to reduce or enhance certain aspects of a signal
** Analogue filter, a basic building block of signal processing much used in electronics
** Filters used in digital image processing

## Arts and entertainment

* Filter (TV series), a show on the G4 channel
* Filter (magazine), an indie music magazine
* Filter Theatre, a British theatre company
* The Filter, a digital content services company based in England
* Filter (band), an American rock band
* "Filter", a song from the album Bend by 8stops7
* "Filter", a song from the album Map of the Soul: 7 by BTS
* "Filter", a song from the album Ruby by Jennie

## Other uses

* Biofilter, a pollution control technique using a bioreactor containing living material to capture and biologically degrade pollutants
* Filtering (housing), the process of housing units becoming more affordable with age
* Category-based filtering of perception, according to (objectivist interpretations of) Kant
** Affective filter, an impediment to learning or acquisition caused by negative emotional ("affective") responses to one's environment
* Lane splitting, a practice that cyclists use to pass slow or stopped congested traffic
* The ability of a person to self-censor themselves (their thoughts) whilst speaking