The Collaborative International Dictionary of English v.0.48:

Diorama \Di`o*ra"ma\, n. [Gr. ? to see through; ? = dia` through
   + ? to see; cf. ? that which is seen, a sight: cf. F.
   diorama. Cf. Panorama.]
   1. A mode of scenic representation, invented by Daguerre and
      Bouton, in which a painting is seen from a distance
      through a large opening. By a combination of transparent
      and opaque painting, and of transmitted and reflected
      light, and by contrivances such as screens and shutters,
      much diversity of scenic effect is produced.
      [1913 Webster]

   2. A building used for such an exhibition.
      [1913 Webster]

WordNet (r) 3.0 (2006):

diorama
    n 1: a picture (or series of pictures) representing a continuous
         scene [syn: panorama, cyclorama, diorama]