Cukoloris

A cukoloris is a flat opaque object with holes in it. It is held in front of a light to make a pattern of shadows on the subject, such as tree branches. It can be constructed out of styrofoam, wood, or cardboard, or if it is meant to be mounted very close to the light, even metal.

The name is often shortened to "cookie," a usage which is probably reinforced by the fact that a cukoloris "cuts holes" in a beam of light much like a cookie cutter cuts cookies from a sheet of dough. A similar device in theater stagecraft is called a "gobo," supposedly short for "goes before optics." A gobo is inserted between the actual lamp and a focusing lens, permitting a much more focused image from the gobo, in the manner of a slide projector. A "cookie" would instead be hung after the lens and produces an indistinct shadow.

The word cukoloris is Gaelic and means "ghost charm." How a Gaelic word became a standard term in film production is unknown. Moreover, there doesn't seem to be any consensus about the spelling. Variations include cucoloris, cuculoris, cucalorus, and kookaloris.

It can also refer to a flickering light source or reflection, to suggest, say, a pile of gold or glowing orb of power lighting up the faces of the actors. The gold itself need not be visible to the audience.

Used in any number of high tech shows and movies in which a computer display seems to be projecting its screen image onto the user's face. It's not entirely accurate, but that's not why they use it in the first place.

Films -- Live-Action

 * 2001: A Space Odyssey, first to evoke the trope with a projector and a computer display.
 * Alien
 * David Lynch's Dune
 * Batman and Robin
 * Swordfish
 * Sucker Punch

Live-Action TV

 * A well known example of the "flickering light source" version is the stargate in the Stargate Verse. To avoid CGI costs the open gate is in many shots offscreen but its flickering light -- produced by a stagehand warping a flexible mirror -- illuminates the rest of the scene.
 * Star Trek: The Original Series: Though not exactly "an opaque sheet with holes in it," shadows from devices like these were often used to suggest structural detail that's off camera (and so doesn't have to actually be built). Look in the "overhead" area of the ship's interiors, particularly where a corridor opens onto a larger junction.
 * Doctor Who:
 * In the episode "The Seeds of Death", the launch countdown is projected onto Gia's face.
 * For their (now-lost) story "The Myth Makers," the BBC used the Trojan horse's shadow, and model shots of the horse, to avoid the cost of building a full-size version.