n.
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.