Another brick in the wall

French philosopher and writer Denis Diderot (1713-1784) proposed the term "fourth wall" in theater and it spread with the advent of theatrical realism.

The fourth wall is the imaginary wall at the front of a three-walled stage, through which the audience sees the action in a play. Even though it is not there, physically, the audience is supposed to assume there is a "fourth wall" present.

The term has been adapted to refer to the boundary between the performance and the audience. American film critic Vincent Canby (1924-2000) described it in 1987 as "that invisible screen that forever separates the audience from the stage".