The ratio of the contrast from the most brightly lit area of the subject to the darkest area
Difference between intensity of key light and fill (shadow) light in a scene.
Studio strobes or flashes are often placed to one side of the subject to prevent flat even lighting. The Lighting Ratio is the ratio of the light from one side of the subject to the other. Light fall-off and shadows cause one side to be illuminated more than another. A pleasant lighting ratio will have the darker side about 1 to 1.5 stops darker than the bright side.