Fourth Wall

The term "fourth wall" stems from the absence of a fourth wall on a three-walled set where the audience is viewing the production. The audience is supposed to assume there is a "fourth wall" present, even though it physically is not there. This is widely noticeable on various television programs, such as situational comedies, but the term originated in theatre, where conventional three-walled stage sets provide a more obvious "fourth wall".

Description stolen from Wikipedia