The wall that separates the stage from the auditorium in a modern theater WordNet 3.0 & Open English WordNet

A proscenium is the metaphorical vertical plane of space in a theatre, usually surrounded on the top and sides by a physical proscenium arch and on the bottom by the stage floor itself, which serves as the frame into which the audience observes from a more or less unified angle the events taking place upon the stage during a theatrical performance. Wikipedia

vertical plane at the front of a theatrical stage Wikidata