A slang term used to refer to the main curtain, house curtain, or grand drape in a proscenium theatre. May fly (guillotine) or travel, usually made of 100% fullness heavy velour. Can be simple or intricate, as the architecture of the space warrants.
Photo from http://www.gerriets.com/en/custom_manufacturing/theater_curtains.php.
Photo from http://www.gerriets.com/en/custom_manufacturing/theater_curtains.php.