The key light is the principle light source used in production. Secondary light sources are called fill lights.
Though the terms high key and low key seem to imply high intensity and low intensity, respectively, the words high and low in this context refer to the position of the light, not the level of illumination. In high key lighting, the key light is situated high above the subject, making for less intense illumination and less light contrast in the shot.
The bright, even lighting produced by the high key light is often used for news broadcasts, musicals and comedies.