Proscenium

The Proscenium is the name of the wall that separates front from back stage in a Proscenium style theater. A proscenium theater is a theater where the audience sits on one side of the action. It typically has an "apron" that brings part of the action out toward the audience in front of the Grand Drape. There may be an orchestra pit in front of or under the apron. A proscenium wall traditionally has an arch opening in it (The proscenium arch). This opening is the Show Portal or window into the world of the show. The proscenium also hides the backstage areas from Audience view. A proscenium theater may or may not have a fly system.

A room set up with all of the audience seats facing one direction (toward the stage) may be considered "proscenium style," regardless of whether there's an architectural arch or not. Technically, if there's no proscenium, the staging genre should be called "end stage."