The fourth wall
Etymology
A theatrical metaphor: a proscenium set has three visible walls, and the open side facing the audience is imagined as a transparent fourth. The idea traces to Enlightenment thinking about theatrical illusion (Diderot), with the explicit "fourth wall" phrasing crystallizing later in stage and film criticism.
noun phrase · /thuh FORTH WAWL/
The imaginary barrier between performers and audience through which the audience watches unseen.
What it names
The invisible convention that lets us watch fiction as if spying on a real world — and gives us the vivid phrase "breaking the fourth wall" for when a character looks straight at us.
Example
The narrator breaks the fourth wall, turning to the camera to explain exactly how he is about to lie.
Related idea
Breaking the fourth wall means a character acknowledging the audience directly; an aside, its theatrical cousin, is a remark the audience hears but other characters supposedly do not. The aside leaks through the wall; breaking it tears the wall down.