pro ‧ sce ‧ ni ‧ um /prəˈsiːniəm, prəʊ- $ prə-, proʊ-/ BrE AmE noun [countable] technical
[ Date: 1600-1700 ; Language: Latin ; Origin: Greek proskenion , from skene 'building in front of which plays are performed' ]
the part of a theatre stage that is in front of the curtain:
the proscenium arch (=the arch over the stage where the curtain can be attached)