The Collaborative International Dictionary of English v.0.48:

backdrop \back"drop\ n.
   1. the scenery hung at back of stage. Also called in Britain
      backcloth.

   Syn: background, backcloth
        [WordNet 1.5]

   2. the background, setting, or circumstances of an event; as,
      the backdrop for the summit meeting.
      [PJC]

WordNet (r) 3.0 (2006):

backdrop
    n 1: scenery hung at back of stage [syn: backdrop,
         background, backcloth]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "backdrop":
   act drop, arena, asbestos, asbestos board, back, background,
   batten, border, cloth, coulisse, counterweight, curtain,
   curtain board, cyclorama, decor, distance, drop, drop curtain,
   field, fire curtain, flat, flipper, ground, hanging, hinterland,
   locale, mise-en-scene, rag, rear, scene, scenery, screen, setting,
   side scene, stage, stage screw, stage set, stage setting, tab,
   tableau, teaser, theater, tormentor, transformation,
   transformation scene, wing, wingcut, woodcut