Backlight in
filmmaking terminology means a light directed at the subject from
behind.
This was used a lot in the great studio ages of black and white and then colour films. It creates almost a halo of light around the silhouette of the subject be it a tough guy’s hat or more often a leading lady’s golden hair.
The usage diminished in the era of location shooting, as it demanded a lot of power and a very fixed positioning style. It is still very much in use now though in studio or controlled environments.
The light is set up behind and above the subject, pointing almost at the camera, just outside the edge of the shot.
An interesting link about the use of lights: http://www.cybercollege.com/tvp031.htm