In theater, film and television, the imaginary wall which would theoretically prevent the audience from seeing the action. When a character turns directly to the audience (or camera) and addresses them, displaying an awareness that they are in a performance, they are breaking the fourth wall.
The fourth wall was well-nigh annihilated, to great effect, in Ferris Bueller's Day Off, when Ferris not only addressed the camera but also turned the fourth wall into a PowerPoint-like presentation on how to feign illness.
John Cusack's character spends most of High Fidelity talking to the camera as if the fourth wall didn't even exist. I half expected him to introduce the audience to everyone in the film, including Sara Gilbert's character. Mmm... Sara Gilbert...
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