A very common fictional concept is that the characters are unaware of the fact that they are characters in somebody else's work of fiction.
This separation between the characters' world and the audience is the Fourth Wall -- named for the imaginary wall at the front of a stage play beyond which the actors are (usually) not supposed to cross.
It's an [[OmnipresentTropes Omnipresent Trope]], because the separation of fiction and audience helps preserve the latter's WillingSuspensionOfDisbelief: The fictional characters treat their story as RealLife, and audience interprets it the same way.
See ThreeWallSet for the production implications of this concept; for example, TheCouch often directly faces the Fourth Wall.
Straight uses of the Fourth Wall are ''far'' too numerous and trivial to list, compared to examples of BreakingTheFourthWall or when there is NoFourthWall.
The exploration and subversion of the Fourth Wall is a common trait of Post Modernism.
Not to be confused with [[WebVideo/AtopTheFourthWall That web show done by Linkara that reviews bad comic books.]]
Big list of related pages (overlaps somewhat with MetafictionDemandedThisIndex):
* FoundFootageFilms: Putting the camera on the character's side of the fourth wall.