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 Omnipresent Trope, because the separation of fiction and audience helps preserve the latter's Willing Suspension of Disbelief: The fictional characters treat their story as Real Life, and audience interprets it the same way.

See Three-Wall Set for the production implications of this concept; for example, The Couch often directly faces the Fourth Wall.

Straight uses of the Fourth Wall are far too numerous and trivial to list, compared to examples of Breaking the Fourth Wall or when there is No Fourth Wall.

The exploration and subversion of the Fourth Wall is a common trait of Post Modernism.

Not to be confused with that web show done by Linkara that reviews bad comic books.

The category overlaps somewhat with Metafiction Demanded This Index.