FOURTHWALL
Fourth wall
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.The above text is a snippet from Wikipedia: Fourth wall
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