Key light

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In lighting, the key light is the main illuminator — in other words, the lighting unit from which the greatest concentration of light falls on the subject.

The secondary light, usually on the other side, at the front, as the key light and typically set to half the power of the key light is the fill light or kicker light.

The amount of light falling on the subject depends on the inverse square law, in other words, it is proportional to the square of the distance between the lighting unit and the subject. What this means is that a light drops by 2 EV when you double the distance from the subject, so an 800 WS light at 8 foot will have the same power (assuming that everything else is identical) as a 200 WS light at 4 foot, or an 800 WS light at 4 foot set to 2 EV lower.