Exposure.

In photography, exposure is the amount of light per unit area reaching a frame of photographic film or the surface of an electronic image sensor, as determined by shutter speed, lens aperture, and scene luminance.

three terms are used to describe exposure in an image. 

Overexposed.     meaning too much light is hitting the film, or in the case of a digital camera, the sensor. Over exposed images don't have much detail and appear to be "washed-out". 

 Underexposed.    when too little light is hitting the film or sensor, the images appear to be dark and "murky". 

Optimal exposure.     Optimal exposure refers to just the right amount of light hitting the film or sensor. 

Often times, split-lighting is used to depict expressions.