The Collaborative International Dictionary of English v.0.48:

Proscenium \Pro*sce"ni*um\, n.; pl. Proscenia. [L., fr. Gr. ?;
   ? before + ? a tent, a wooden stage, the stage. See Scene.]
   1. (Anc. Theater) The part where the actors performed; the
      stage.
      [1913 Webster]

   2. (Modern Theater) The part of the stage in front of the
      curtain; sometimes, the curtain and its framework.
      [1913 Webster]
      [1913 Webster]

WordNet (r) 3.0 (2006):

proscenium
    n 1: the part of a modern theater stage between the curtain and
         the orchestra (i.e., in front of the curtain) [syn:
         proscenium, apron, forestage]
    2: the wall that separates the stage from the auditorium in a
       modern theater [syn: proscenium, proscenium wall]