In optics, an aperture is a hole or an opening through which light is admitted. In photography, aperture refers to the diameter of the aperture stop rather than the physical stop or the opening itself.
The aperture stop of a photographic lens can be adjusted to control the amount of light reaching the film or image sensor. In combination with shutter speed, the aperture size will regulate the film's degree of exposure to light. Typically, a fast shutter speed will require a larger aperture to ensure sufficient light exposure, and a slow shutter speed will require a smaller aperture to avoid excessive exposure.
The lens aperture is usually specified as an f-number, the ratio of focal length to effective aperture diameter. A lens typically has a set of marked "f-stops" that the f-number can be set to. A lower f-number denotes a greater aperture opening which allows more light to reach the film.