1) In computer programming, a filter is a program or section of code that is designed to examine each input or output request for certain qualifying criteria and then process or forward it accordingly. This term was used in UNIX systems and is now used in other operating systems. A filter is "pass-through" code that takes input data, makes some specific decision about it and possible transformation of it, and passes it on to another program in a kind of pipeline. Usually, a filter does no input/output operation on its own. Filters are sometimes used to remove or insert headers or control characters in data.
In Windows operating systems, using Microsoft's Internet Server Application Programming Interface ( ISAPI ), you can write a filter (in the form of a dynamic link library or DLL file) that the operating system gives control each time there is a Hypertext Transport Control ( HTTP ) request. Such a filter might log certain or all requests or encrypt data or take some other selective action.
2) In telecommunications, a filter is a device that selectively sorts signals and passes through a desired range of signals while suppressing the others. This kind of filter is used to suppress noise or to separate signals into bandwidth channels.
3) In Photoshop and other graphic applications, a filter is a particular effect that can be applied to an image or part of an image. Filters can be fairly simple effects used to mimic traditional photographic filters (which are pieces of colored glass or gelatine placed over the lens to absorb specific wavelengths of light) or they can be complex programs used to create painterly effects.