Backdrop

Backdrop refers to a large photographic backing or painting for the background of a scene (i.e., a view seen outside a window, a landscape scene, mountains, etc.). It’s usually painted on flats (composed of plywood or cloth). 

A large curved backdrop (often illustrating the sky) is known as a cyclorama. Backdrops were more common before the current trend toward on-location shooting and the use of bluescreens.