The Collaborative International Dictionary of English v.0.48:

Greenroom \Green"room`\ (gr[=e]n"room`), n.
   The retiring room of actors and actresses in a theater.
   [1913 Webster]

WordNet (r) 3.0 (2006):

greenroom
    n 1: a backstage room in a theater where performers rest or have
         visitors

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "greenroom":
   L, R, acting area, antechamber, anteroom, apron, apron stage,
   audience chamber, backstage, band shell, bandstand, board, bridge,
   coulisse, dock, dressing room, flies, fly floor, fly gallery,
   forestage, grid, gridiron, lightboard, lounge, orchestra,
   orchestra pit, performing area, pit, presence chamber, proscenium,
   proscenium stage, reception room, shell, side room, stage,
   stage left, stage right, switchboard, the boards, throne room,
   waiting room, wardroom, wings