fourth wall

/F AO1 R TH W AO0 L/
noun
  1. 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. 2

    (by extension) The boundary between the fiction and the audience.

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Source: https://en.wiktionary.org/wiki/fourth%20wall