Barn Doors
Four hinged flaps mounted on the front of a light to cut its beam into a rectangle.
In depth
Barn doors let each edge of a beam be trimmed independently, which is how a light can be kept off a background, off a lens, or off part of a set without moving it. They work best on hard sources, since a soft source's light escapes around the flaps.
Examples in practice
Closing the top door of a rim light stops it flaring straight into the lens while keeping the shoulder edge lit.