Proscenium
A proscenium is the area of a theatre surrounding the stage opening. A proscenium arch is the arch over this area.
In ancient Rome, the stage area in front of the scaenae frons was known as the "proscenium", meaning "in front of the scenery". In the Roman theater, no proscenium arch existed, in the modern sense. However, Roman theaters were similar to modern proscenium theaters in the sense that the entire audience had a restricted range of views on the stage all of which were from the front, rather than the sides or back.