Definitions for "Key light"
Principal source of illumination. Usually a spotlight.
(Esp. TV & Film lighting) The dominant light source/direction in a lighting state. In a sunny drawing room, the key light would be through the window, for a naturalistic exterior scene the direction of the key light could change as the sun progressed across the sky. See Fill Light.
Also "main light" -- the source providing the primary illumination and sense of the direction of the light