Cucoloris - A filter that casts patterned light. They're apparently often just pieces of cardboard or other materials with patterns removed. Also called a cookie by people in the movie biz. I couldn't find any pictures of a cucoloris, but here's a photographer's example of using a potted palm tree as an improvised cucoloris to make a scene more interesting.

Via Roger Ebert's glossary of movie terms. And note that he spells it differently.

Previous WOTD - Ceteris paribus