The amount of light or darkness in a photograph is known as exposure. In the domain of photography, exposure refers to the amount of light based on a unit area that reaches the photographic film. In digital cameras, this light falls on an image sensor instead.
The exposure of a photograph is determined by the aperture of the lens, the shutter speed, and the luminance of the scene. The exposure itself, is measured in terms of lux seconds. As a jargon in the domain of photography, one exposure is a single cycle of the shutter. It is like the backbone of getting good quality images.