The proscenium of a theater stage is a structure in front of the stage that frames the action of the play. It can be square or arched, and the stage curtain is generally directly behind it.
The ancient Greeks gave us the modern concept of theater and, with it, the proscenium, one of the divisions of the stage. The word itself can be broken down into prō-, “in front of,” and skēnē, “scene”: The proscenium is thus the part “in front of the scenery.” In a modern theater, the proscenium makes up the so-called “fourth wall,” the invisible barrier that separates the stage from the audience and through which the audience watches the action.
the part of a modern theater stage between the curtain and the orchestra (i.e., in front of the curtain)
the wall that separates the stage from the auditorium in a modern theater