Used in photography to define the size of the opening in the lens, which in non-basic cameras can be adjusted to control the amount of light reaching the film. In combination with variation of the shutter speed, this will regulate the degree to which the film is exposed to light. Typically, a fast shutter speed will require a larger aperture to ensure that the film receives sufficient exposure to light, just as a slow shutter speed will typically require a smaller aperture to prevent the film from receiving excessive exposure to light.
A device called diaphragm controls the aperture. The diaphragm can be considered to function much like the pupil of the eye, it controls the effective diameter of the lens opening. Reducing the aperture size increases the depth of field, which describes the extent to which subject matter lying closer than or farther from the actual plane of focus appears to be in focus. In general, the smaller the aperture, the greater the distance from the plane of focus the subject matter may be while still appearing in focus.
Aperture is usually measured in f-numbers. A lens will have a set of "f-stops" that represent doublings in the amount of light let through the aperture. A lower f-stop number denotes a greater aperture opening which allows more light to reach the film. A typical standard lens will have an f-stop range from f/16 (very small aperture) to f/2 (large aperture). Professional lenses can have f-stops as low as f/1.2 (very large aperture). These are known as "fast" lenses because they allow much more light to reach the film and therefore reduce the exposure time of the film. These lenses are favored especially by photojournalists as they often work in dim light, have no opportunity to introduce supplementary lighting, and capture fast breaking events.
In computer displays.