The fourth wall is a performance convention in which an invisible, imaginary wall separates actors from the audience. While the audience can see through this "wall", the convention assumes the actors act as if they cannot. From the 16th century onward, the rise of illusionism in staging practices, which culminated in the realism and naturalism of the theatre of the 19th century, led to the development of the fourth wall concept.

The metaphor suggests a relationship to the mise-en-scène behind a proscenium arch. When a scene is set indoors and three of the walls of its room are presented onstage, in what is known as a box set, the fourth of them would run along the line (technically called the proscenium) dividing the room from the auditorium. The fourth wall, though, is a theatrical convention, rather than of set design. The actors ignore the audience, focus their attention exclusively on the dramatic world, and remain absorbed in its fiction, in a state that the theatre practitioner Konstantin Stanislavski called "public solitude" (the ability to behave as one would in private, despite, in actuality, being watched intently while so doing, or to be 'alone in public'). In this way, the fourth wall exists regardless of the presence of any actual walls in the set, the physical arrangement of the theatre building or performance space, or the actors' distance from or proximity to the audience. In practice, performers often feed off the energy of the audience in a palpable way while modulating performance around the collective response, especially in pacing action around outbursts of laughter, so that lines are not delivered inaudibly.

Breaking the fourth wall is violating this performance convention, which has been adopted more generally in the drama. This can be done by either directly referring to the audience, the play as a play, or the characters' fictionality. The temporary suspension of the convention in this way draws attention to its use in the rest of the performance. This act of drawing attention to a play's performance conventions is metatheatrical. A similar effect of metareference is achieved when the performance convention of avoiding direct contact with the camera, generally used by actors in a television drama or film, is temporarily suspended. The phrase "breaking the fourth wall" is used to describe such effects in those media. Breaking the fourth wall is also possible in other media, such as video games and books.