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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 a 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. This can also occur in literature and video games when a character acknowledges the reader or player.
Deadpool breaks the fourth wall many times in his new video game. One example includes:

Wolverine: What were you doing over there?
Deadpool: Just a little surprise for our player!
by DropTheB8ss February 13, 2014
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Nov 23 Word of the Day
A massive dump that takes 10 to 15 times to flush.
After Thanksgiving, I had to Take a Trump.
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The fourth wall is usually in reference to video games, in which the main character(s) are making reference to (or sometimes speaking directly to) the player(s).
Dammit, Monika’s gone and broken the Fourth Wall yet again.”
via giphy
by pheonix8905 July 22, 2019
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