The thin line that exists between a story and reality. When a character in a story tells the reader in some way that they know that they are a character in a story, that is called 'breaking the fourth wall'.
Character 1: The author is a jackass who can't write me properly.
Crashing noise
Character 2: Well, there goes the fourth wall.
by Kyo Kusanagi July 7, 2003
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Something that is frequently broken by Deadpool
Deadpool: *gasp* A fourth wall break inside a fourth wall break. That’s like, sixteen walls.
by YourTypicalTeenager April 27, 2018
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A wall that is always being broken so reality meets fantasy. Like in Anime and Cartoons.

Anime like Naruto, Pokémon, Bobobo-bo Bo-bobo and Duel Masters break the fourth wall.

Cartoons Ed, Edd n Eddy, Powerpuff Girls, Brandy and Mr. Whiskers and Yin Yang Yo break the fourth wall.

They bring up the word "episode" sometimes, too.

They look at YOU directly and they mention their own show!
Every Anime and Cartoon has a fourth wall... It's just that the directors and producers don't have them break it. It's not in their script.
by AnimeandToonLover August 7, 2009
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The Fourth Wall is an imaginary "wall" that separates you from the game or movie. This Fourth Wall is If someone or something addresses you; the player or the viewer, directly or indirectly, or when something that is typical for a particular kind of movie or game is addressed, it is known as breaking the fourth wall.
Deadpool: "Superhero landing! She's gonna do it!"
Negasonic Teenage Warhead: *does it*
Deadpool: "Wooo! Superhero landing. That's hard on the knees!"
The Fourth Wall just broke.
by Shadvox May 14, 2018
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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 14, 2014
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In theatre, the side or sides of the set facing the audience that have no physical reality. The audience and actors aknowledge this deliniation as a convention of theatre, unless the director chooses to "break" the fourth wall.
by Andrew January 20, 2005
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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.”
by pheonix8905 July 22, 2019
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