A key light is the primary source of illumination in a scene, used in photography and film to highlight the subject. It creates depth and dimension by casting shadows, which helps to define the subject's features. The intensity and angle of the key light can significantly affect the mood and tone of the image. In a typical three-point lighting setup, the key light is complemented by a fill light and a backlight. The fill light softens shadows created by the key light, while the backlight adds separation between the subject and the background. Together, these elements enhance the overall visual composition.