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The fourth wall is a performance convention in which an invisible, imaginary wall separates actors from the audience. While the audience can see through this wall, the convention assumes the actors act as if they cannot. From the 16th century onward, the rise of illusionism in staging practices, which culminated in the realism and naturalism of the theatre of the 19th century, led to the development of the fourth wall concept.
Breaking the fourth wall is violating this performance convention, which has been adopted more generally in the drama. This can be done by either directly referring to the audience, the play as a play, or the characters' fictionality. The temporary suspension of the convention in this way draws attention to its use in the rest of the performance. This act of drawing attention to a play's performance conventions is metatheatrical. A similar effect of metareference is achieved when the performance convention of avoiding direct contact with the camera, generally used by actors in a television drama or film, is temporarily suspended. The phrase "breaking the fourth wall" is used to describe such effects in those media. Breaking the fourth wall is also possible in other media, such as video games and books.
https://en.wikipedia.org/wiki/Fourth_wall