Also called proscenium arch.the arch that separates a stage from the auditorium. Abbreviation:pros.
2.
(formerly) the apron or, esp. in ancient theater, the stage itself.
Origin: 1600–10; < L proscēnium, proscaenium < Gk prosknion entrance to a tent, porch, stage (LGk: stage curtain), equiv. to pro-pro-2+ skēn() (see scene) + -ion neut. n. suffix
[proh-see-nee-uh
m, pruh-] 
nion