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 and defining shapes, making the subject stand out. 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 back light. The fill light softens shadows created by the key light, while the back light adds separation between the subject and the background, enhancing the overall visual composition.