proscenium

thumb|The proscenium arch of the theatre in the Auditorium Building (Chicago)|Auditorium Building in Chicago. The proscenium arch is the frame decorated with square tiles that forms the vertical rectangle separating the stage (mostly behind the lowered curtain) from the auditorium (the area with seats).|alt=Interior view of a theater.

A proscenium (, ) is the virtual vertical plane of space in a theatre, usually surrounded on the top and sides by a physical proscenium arch (whether truly "arched" or not) and on the bottom by the stage floor itself, which serves as the frame into which the audience observes from a more or less unified angle the events taking place upon the stage during a theatrical performance. The concept of the fourth wall of the theatre stage space that faces the audience is essentially the same.