Noun
(theater) The division between the fictional world depicted, and the actual actors, props, etc.
(art) The divide between critics or audience members and author or theater practitioner.
(theater) Any of various other divisions between groups of people in the theatre.
(decorating) The ceiling.
(decorating) The floor.
Used other than figuratively or idiomatically: see fifth, wall.
Source: en.wiktionary.orgThe role of television is the illusion of company, noise. I call it the fifth wall and the second window: the window of illusion. Mumia Abu-Jamal