The fourth wall is the imaginary wall at the front of the stage in a proscenium theater, the term stems from the absence of a fourth wall on a three-walled set where the audience is viewing the production, through which the audience sees the action in the world of the play. The meaning of the term "fourth wall" has been adapted to refer to the boundary between the fiction and the audience. "Fourth wall" is part of the suspension of disbelief between a fictional work and an audience, an invisible barrier between realities.
"Breaking the fourth wall" in general refers when a character is showing his/her awareness of the audience, ignoring the barrier between realities to directly interact with them. Generally this is done for comedic effect. The level of fourth wall breaking varies between characters and writers, with some merely having knowledge from our reality that a fictional character wouldn't/couldn't know (referring to Tobey Maguire as Spider-Man) through to attempting to influence the story by pleas to the writer and/or artist (like Deadpool trying to kill the creators of a comic he appeared in by travelling to their dimension).
Deadpool is the most famous character in the Marvel universe to have broken this wall frequently over and over again. Loki has also broken the 4th wall as well.
(See Also: Blonde Phantom, Deadpool, She-Hulk)
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