fourth wall

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The space separating the audience from the action of a theatrical performance, traditionally conceived of as an imaginary wall completing the enclosure of the stage.

Wiktionary

1. n. performing arts, idiomatic 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. n. by extension The boundary between the fiction and the audience.