The fourth wall is the imaginary invisible wall at the front of the stage in a proscenium theater, through which the audience sees the action in the world of the play. Although the concept has been around since before the ancient Greeks and is used by Shakespeare, the phrase itself is generally presumed to have originated in nineteenth century theatre with the advent of theatrical realism. Critic Vincent Canby described it in 1987 as "that invisible screen that forever separates the audience from the stage."

In A.R. Gurney's The Fourth Wall, a play originally produced in the United States in regional theatre, a quartet of characters deal with housewife Peggy's obsession with a blank wall in her house, slowly being drawn into a series of theatre clichés as the furniture and action on the stage become more and more directed to the supposed fourth wall.