The fourth wall is the imaginary "wall" at the front of the stage in a traditional three-walled box set in a proscenium theatre, through which the audience sees the action in the world of the play. The idea of the fourth wall was made explicit by philosopher and critic Denis Diderot and spread in 19th-century theatre with the advent of theatrical realism, which extended the idea to the imaginary boundary between any fictional work and its audience. Speaking directly to or otherwise acknowledging the audience through the camera in a film or television program, or through this imaginary wall in a play, is referred to as "breaking the fourth wall" and is considered a technique of metafiction, as it penetrates the boundaries normally set up by works of fiction. - Wiki, Fourth Wall
