fourth wall

noun2

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.

   • There's been a convention in the theater world to think of the division between audience and spectacle as a fourth wall, a wall that the playwright tries to eliminate through the force of his drama. — 2005
2. •
   The boundary between the fiction and the audience.

   • The fourth wall is the imaginary barrier between the stage and the audience, and the phrase is a metaphor for the dramatic frame. — 2003