1. A gaffer in the motion picture industry and on a television crew is an electrician, sometimes head of the electrical department, responsible for the execution (and sometimes the design) of the lighting plan for a production.
Gaffer within the motion picture industry originally related to the moving of overhead equipment to control lighting levels using a gaff. It has been used for the chief electrician in films since 1936. His assistant is the best boy.
Experienced gaffers can coordinate the entire job of lighting, given knowledge of the time of day and conditions to be portrayed, managing resources as broad as electrical generators, lights, cable, and manpower. Gaffers are responsible for knowing the appropriate color of gel (plastic sheeting) to put on the lights or windows to achieve a variety of effects, such as transforming midday into a beautiful sunset. They can re-create the flicker of lights in a subway car, the motion of light inside a turning airplane, or the passage of night into day.
Usually, the gaffer works for and reports to the Director of Photography (the DP or DOP).