Sentence:  There's been a convention in the theater world to think of the division between audience and spectacle as a <WSD>fourth wall</WSD>, a wall that the playwright tries to eliminate through the force of his drama. 
Meaning of the word marked with <WSD> tags:  The imaginary invisible wall at the front of the stage in a proscenium theatre, through which the audience sees the action in the world of the play.