Wiktionary
n. 1 (context performing arts idiomatic English) The imaginary invisible wall at the front of the stage in a proscenium theatre, through which the audience sees the action in the world of the play. 2 (context by extension English) The boundary between the fiction and the audience.
Wikipedia
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 concept is usually attributed to the philosopher, critic and dramatist Denis Diderot. The term itself was used by Molière. The fourth wall illusion is often associated with naturalist theatre of the mid 19th-century, and especially with the innovations of the French director André Antoine.
The restrictions of the fourth wall were challenged in 20th-century theatre. Speaking directly to, otherwise acknowledging or doing something to the audience through this imaginary wall – or, in film, television, and video games, through a camera – is known as "breaking the fourth wall". As it is a penetration of a boundary normally set up or assumed by works of fiction, this is considered a metafictional technique. In literature and video games, it occurs when a character acknowledges the reader or player.
Breaking the fourth wall should not be confused with the aside or the soliloquy, dramatic devices often used by playwrights where characters on stage are delivering inner monologues, giving the audience insight into their thoughts.
Fourth Wall is the second studio album by English rock band The Flying Lizards, released in 1981 by record label Virgin. The album features numerous collaborators, including Robert Fripp.
Usage examples of "fourth wall".
I turned to the fourth wall, over which a long curtain was hanging.
His end looked like the set of a cable-access show: a paneled room with a missing fourth wall.
Three walls crowded with heavily framed assortments of fruit, hunt still lifes, peasant scenes -- and the fourth wall is glazed and sky-bright.
The fourth wall, the one I faced as I slipped between some crates, had been formed by another warehouse.
On the fourth wall facing east was a map of Rathillien in colored glass, all Kendar work, of course: the Highborn were about as artistically inept as an intelligent race could be.
My preference is for the old, traditional form of dramatic presentation known as the proscenium theater: three walls, and an imaginary fourth wall between the players and the audience.
Over the centuries many methods have been used to break that fourth wall for various reasons.
The poster which had hung on the fourth wall had been torn down, leaving pockmarks in the plaster, like bullet holes.
We could do the houses first, but I think we'll save some effort by using the palisade as the fourth wall for some of them, and for the grainmill, when we build it.
The fourth wall the outside wall, was cement padded and painted a restful shade of green.
Reacher climbed down the fourth wall and walked back to her stall.
Opalescence swirled like slow smoke in walls and ceilings, except for the fourth wall, which stood open on a balcony.