Is it ensure, insure, or assure?
in theatre, background device employed to cover the back and sometimes the sides of the stage and used with special lighting to create the illusion of sky, open space, or great distance at the rear of the stage setting. Introduced early in the 20th century, a cyclorama usually forms a smoothly curving, concave wall at the rear of the stage. Some, called dome horizons, also curve at the top, heightening the illusion of open space. Although some theatres have a curved back wall that serves as a permanent cyclorama, it most commonly consists of a drop curtain that can be raised or lowered according to need.