A scrim is a very light textile made from cotton, or sometimes flax. Its light weight and translucence means it is often used for making curtains. The fabric can also be used for bookbinding and upholstery.
Scrims have also seen extensive use in theatre, Sharktooth scrim, is the more proper term to describe it, where they are often used for special effects. In theater a scrim can refer to any such thin screen, and are made out of a wide variety of materials. The advantage of scrims is that when lit from the front they can appear opaque to the audience, until a quick change in stage lighting (lighting from behind) makes them almost fully transparent.