Definitions courtesy of "The Set Lighting Technician's Handbook" (yes, we have a handbook, and you can buy it here).
Best Boy: The assistant chief lighting technician, or second electrician. The best boy is the gaffer's assistant, and is responsible for hiring electricians, ordering equipment, and generally running the show so that all the gaffer has to do is light the set.
Boned: Pronounced like "loaned". To have something or someone impede your ability to do your job - "You boned me by putting that shit right where the light has to go." or "How's your day going? We're so boned, you don't even want to know."
C-47: Also called "bullets". A common everyday wooden clothespin (the rumor is that it's called a C-47 because that was the only way to get the producers to pay for them). C-47's are used to hold gels or diffusion to doors of lights.
Color Temperature: A temperature expressed in degrees Kelvin (K) that defines the color makeup of light emitted by a source, such as the sun or a filament lamp, which has a continuous color spectrum.
Cyclorama (Cyc): A seamless set piece - normally permanently installed on a stage, usually white, and curved where it meets the floor. It is used to create a limbo background, having no discernible horizon or texture. If you drive a lift onto the curved part of the cyc where it meets the floor (the 'foot'), you crack the cyc and cost production a lot of money.
DP (Director of Photography): The person in charge of lighting and camera departments. The DP is the ultimate authority on how the scene is lit, where the camera goes, how the camera moves, and which lenses and film to use.
Diffusion: Material used in front of lighting fixtures to soften the light they produce. Common types of diffusion (from thickest to thinnest) are Grid Cloth, 216, 250, and Opal
Dikes: Wire cutters
Distant Location: A location that is far enough away from the production's town of origin that the crew must stay overnight.
Distribution Box (Distro box): An electrical box with circuit protection, used to stepdown cable size and connector size and to provide a variety of sized of outlets. Distribution boxes are collectively referred to as "Distro".
Gaffer: The head of the lighting crew. The Gaffer works directly with the Director of Photography. The way I try to explain it is this: The DP tells the Gaffer what he/she wants the scene to look like, and the Gaffer is the one who actually determines what lamps will be used and precisely where they'll go.
Grip Clip: A metal spring clamp. They come in 1", 2", and 3" widths. The 3" ones are a bitch to hold open if you have small hands.
Head: A lighting fixture. "Put all those heads in the truck"
Jockey Boxes: Metal storage containers on the underside of a truck. We usually put cable, distro and such in the jockey boxes.
Kelvin: A unit of measurement of temperature (0 degrees kelvin = -273 degrees Centigrade). In set lighting, the term refers to the color temperature or light and not to it's physical temperature. 3200 degrees Kelvin is tungsten (standard lightbulb color - orangey).
Kick out: The accidental unplugging of a light.
Magic Hour: The hour of light just at sunset, during which the sky creates a beautiful light. Magic hour, in reality, only lasts a few moments.
Martini: The last shot of the day.
Turnaroud: The time between when you go off the clock on one day and call time the next day. 10 hours is legal, but sucks. Crews HATE 10 hour turnaround.
I try my best not to lapse into 'insider jargon' here, but if I do use a term in a post and you don't know what it means, feel free to ask - in the comments or via email (randomblogmail[at]yahoo dot com).