Filter
From Nikonians Wiki - FAQs, Photo Glossary, Good Photo Locations, Help
Transparent lens attachment used to modify the light coming into a lens, to change the color, or other characteristics of an image. They are used both on the camera and in the darkroom.
By extension, some of the mathematical transformations and algorithms which can be applied to an image while postprocessing are referred to as filters, particularly when they emulate filters that would otherwise have been placed over the lens.
Further reading from Nikonians guides:
What filter system [1] Selecting a screw-in or a slide-in system or a mix
UV, Haze and Skylight filters [2] Protecting your lens and getting sharper images
Neutral density and color graduated filters [3] Getting the right exposure under tricky light
Diffusion - Softening filters [4] Making people look better
Rotating filters and hoods [5] A solution to using rotating filters with modern deep scalloped hoods
