A gaffer in the motion picture industry is the head of the electrical department, responsible for the execution (and sometimes the design) of the lighting plan for a production. Sometimes the gaffer is credited as Chief Lighting Technician. In television the term Lighting Director is often used, but sometimes the Technical Director (T.D.) will light the studio set.
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 generators, lights, cable, 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). The DP is responsible for the overall lighting design, but he or she may give a little or a lot of latitude to the gaffer on these matters, depending on their working relationship. The gaffer works with the key grip, who is in charge of some of the equipment related to the lighting. The gaffer will usually have an assistant called a best boy and, depending on the size of the job, crew members who are called "electricians", although not all of them are trained as electricians in the usual sense of the term.
Early studios were "available light" only, so there were articulated mirrored panels in the roof of the studio buildings that could be pushed from the floor by long "gaff" poles to bounce the sunlight to where it was needed on the set. Because the Earth moves continuously these hinged panels would need to be gaffed after each take. Once electric lighting instruments became the standard equipment, the light operators were known as electricians while the older, more experienced lighting technicians were still known as gaffers. Eventually it came to mean someone in charge of lighting.