The Collaborative International Dictionary of English v.0.48:
barndoor \barndoor\ n. An opaque adjustable flap on a lamp fixture; used in photography to cut off light from particular areas. [WordNet 1.5]WordNet (r) 3.0 (2006):
barndoor n 1: an opaque adjustable flap on a lamp fixture; used in photography to cut off light from particular areas