fourth wall

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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.
• noun by extension The boundary between the fiction and the audience.