The fourth wall is a conceptual barrier between those presenting some kind of a communication and those receiving it.
The term originated in the theater, where it refers to the imaginary wall at the front of the stage separating the audience from the performers. The fourth wall, along with the sides and back of the standard stage, encloses the created world of the play.
Breaking the fourth wall means doing or saying something that either explicitly or implicitly acknowledges the artificiality of the environment and the fact that both the presenters and audience are aware of that artificiality. In the theatre, for example, an actor might break the fourth wall physically by walking down from the stage, through the audience and out the door instead of exiting stage left or right. Conceptually, an actor might break the fourth wall by making a reference to the fact that he is a performer in a play, by addressing the audience directly, or by responding to something that happens in the audience, such as a crying baby or a ringing cell phone.
- A PowerPoint slide that mentions death by PowerPoint -- a reference to the fact that these presentations can be almost fatally boring.
- A commercial that refers to the fact that its purpose is to get the viewer to purchase the advertiser's product.
- A character in a video game who asks the player about his scores in other games.
- An interviewee who, when asked what type of salary they're looking for, says "I'd prefer you to name your best offer first. Of course I want as much money as I can get."