An item common in the film and video world, with some crossover into live theatrics. Similar in application to a break up gobo as used in an ellipsoidal, a cucoloris is a piece of equipment designed to be placed in front of a high wattage light source (typically a Fresnel or open face light) to break up the bean into uneven, random patterns. They can be home made or bought from a supplier, and both hard and soft versions exist. Called cookie as slang/for short.
http://www.msegrip.com/matthews-cucoloris.html
http://www.msegrip.com/matthews-cucoloris.html