Backlighting refers to the process of illuminating the subject from the back. In other words, the lighting instrument

Stage lighting instrument

Stage lighting instruments are used in stage lighting to illuminate theatrical productions, concerts, and other performances taking place in live performance venues. They are also used to light television studios and sound stages.Many stagecraft terms vary between the United States and the United...

 and the viewer

Viewer

Things known as a viewer include:* For television viewer, see Audience; Audience measurement; Nielsen ratings* File viewer, a piece of computer software that displays the data stored in a computer file in a human-friendly form...

 are facing towards each other, with the subject in between. This causes the edges of the subject to glow, while the other areas remain darker. The backlight can be a natural or artificial source of light. When artificial, the back light is usually placed directly behind the subject in a 4-point lighting setup.

A back light, which lights foreground elements from the rear, is not to be confused with a background light

Background light

The background light is used to illuminate the background area of a set. The background light will also provide separation between the subject and the background. In the standard 4-point lighting setup, the background light is placed last and is usually placed directly behind the subject and...

, which lights background elements (such as scenery).

In the context of lighting design, The back light is sometimes called hair or shoulder light, because when lighting an actor

Actor

An actor is a person who acts in a dramatic production and who works in film, television, theatre, or radio in that capacity...

 or an actress, backlighting will cause the edges of his or her hair to glow if he or she has fuzzy hair.