Literally, before the scenery The stage or playing area in front of the scenery The arch above the stage is also called the proscenium arch
The part of the stage in front of the curtain; sometimes, the curtain and its framework
(Erika Foss) A proscenium is the part of the modern stage that is in front of the curtain; it is the part of the stage where actors stand to be seen by the audience when the curtain is closed
A proscenium or a proscenium arch is an arch in a theatre which separates the stage from the audience. the part of a theatre stage that is in front of the curtain (proskenion, from skene ). In a theatre, the frame or arch separating the stage from the auditorium, through which the action of a play is viewed. In ancient Greek theatres, the proskenion was an area in front of the skene that eventually functioned as the stage. The first permanent proscenium in the modern sense was built in 1618 at the Farnese Theatre in Parma. Though the arch contained a stage curtain, its main purpose was to provide a sense of spectacle and illusion; scene changes were carried out in view of the audience. Not until the 18th century was the curtain commonly used to hide scene changes. The proscenium opening was of particular importance to 19th-century realist playwrights, for whom it served as a picture frame or an invisible wall through which the audience experienced the illusion of spying on the characters
An arch that frames a box set and holds the curtain, thus creating a sort of invisible boundary through which the audience views the on-stage action of a play
the part of a modern theater stage between the curtain and the orchestra (i e , in front of the curtain)