The proscenium arch is, in literal terms, the square frame around a raised stage area in traditional theatres. It presents a style of theatre which has persisted since Greek times and has as such as become an almost derrogatory term to many modern dramatists.

The proscenium arch perpetuates the fourth wall concept within theatre, which implies that the characters performing on stage are doing so in a four-walled environment, with the "wall" facing the audience being invisible.

Modern theatre attempts to do away with the fourth wall concept and so the proscenium arch concept has fallen out of favour. Modern theatres are designed with the likes of circular stage areas surrounded by the audience, thereby doing away completely with the concept.

The term proscenium comes from the Greek - "pro" - before, "scena" - scenery, because the proscenium is normally in front of the scenery.

See also safety curtain, fourth wall.