barn door

noun

the door of a barn
informal a target so large that it cannot be missed
photog television theatre an adjustable flap over the front of a studio or theatre lamp
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Examples from the Web for barn door