Stop

This is the old-fashioned term for one unit of exposure value, or 1EV using today's terminology. The principle of exposure calculations is that each step in shutter speed or lens aperture (each 'stop') corresponds to a doubling or a halving of the light value. In this way it's easy to play off shutter speed against lens aperture for creative photographic effects. You can also check different parts of the scene for brightness, and work out, for example, that the ground is two 'stops' darker than the sky. This is useful for more advanced exposure techniques and calculations. Photographers still talk in terms of 'stops' even though the correct term would be 'EV values'.