Definition of fourth wall in English:

fourth wall

noun

the fourth wall
  • 1The space which separates a performer or performance from an audience.

    1. 1.1The conceptual barrier between any fictional work and its viewers or readers.
      ‘he breaks the fourth wall by having Sam refer to the script and the play he's acting in’

Origin

Originally used of the proscenium opening in a theatre through which the audience sees the action of a play.