Prose craft
Fourth wall
The imaginary wall separating performers from audience; a narrative convention whose deliberate breaking is metafiction.
The fourth wall is "a common convention in narrative drama in which a metaphorical, invisible, or imaginary wall separates performers" from the audience: the audience looks through it into the story, while the performers behave as if no one were there. The concept grew out of sixteenth-century stage illusionism and matured in nineteenth-century realism; Denis Diderot in 1758 told actors to "imagine a huge wall across the front of the stage," and Leigh Hunt coined the phrase itself in 1807. Breaking it, by addressing the audience or acknowledging one's own fictionality, is metatheatre or metafiction, used for humor or shock, from Groucho Marx's asides to games like The Stanley Parable.
In prose the wall sits between narrator and reader, and every direct address is a choice about point of view. Web fiction plays with the convention constantly: the author's notes around a web serial chapter are a sanctioned place for the wall to come down, which keeps the story itself free to hold it up.