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

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

Social media
Filter (social media), an appearance-altering digital image effect for social media
Beauty filter, a social media effect specifically meant to increase physical attractiveness

Other uses
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

See also
Filter coffee, a method for preparing coffee
Filter feeder, an animal that strains food particles from water
Filtration camp (disambiguation)
Lifter (signal processing)
Philtre (disambiguation)
Separation process
Ultrafilter