proscenium arch

In theatrical design, the arch that frames a stage, separating it from the auditorium.

proscenium arch [prŏ‐seen‐iŭm], the structure separating the main acting area from the auditorium in most Western theatres of the 19th and early 20th centuries. It usually forms a rectangular ‘picture frame’, the ‘picture’ being revealed by opening a curtain. Its associated dramatic conventions often involve the illusion of looking into a room through an invisible ‘fourth wall’.

An arch or any equivalent opening in the proscenium wall through which the stage is seen by the audience.

The noun has one meaning:

Meaning #1: the arch over the opening in the proscenium wall