proscenium vs proscenium arch

proscenium and proscenium arch both are nouns.

Word	Noun	Adjective	Verb	Adverb
proscenium	Yes	No	No	No
proscenium arch	Yes	No	No	No

proscenium (noun)	proscenium arch (noun)
the wall that separates the stage from the auditorium in a modern theater	the arch over the opening in the proscenium wall
the part of a modern theater stage between the curtain and the orchestra (i.e., in front of the curtain)