Fourth Wall: Derived from theatre (Denis Diderot) as the transparent boundary which separates the characters and plot from the audience (Stevenson, 1995). In roleplaying games, it serves as the barrier between the player and character. For example, the things known by the player may not necessarily be known by the character.
Source: Stevenson, J. (1995). The fourth wall and the third space. New York, NY: Center for Playback Theater.