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]

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "proscenium":
   L, R, acting area, anteriority, apron, apron stage, backstage,
   band shell, bandstand, board, bold front, brave face, brave front,
   bridge, coulisse, display, dock, dressing room, facade, face,
   facet, facia, flies, fly floor, fly gallery, fore, forefront,
   foreground, forehand, foreland, forepart, forequarter, foreside,
   forestage, foreword, front, front elevation, front man,
   front matter, front page, front view, frontage, frontal, frontier,
   frontispiece, greenroom, grid, gridiron, head, heading, lap,
   lightboard, obverse, orchestra, orchestra pit, performing area,
   pit, preface, prefix, priority, proscenium stage, shell, stage,
   stage left, stage right, switchboard, the boards, window dressing,
   wings