fourth wall

Meanings

1. 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. The boundary between the fiction and the audience.

Etymology

A reference to the three walls of a box set, with the fourth wall being the imaginary wall separating the performers from the audience. Coined by philosopher and art critic Denis Diderot in 1758 and thus a calque of French quatrième mur.