The Collaborative International Dictionary of English v.0.48:

Diptych \Dip"tych\, n. [L. diptycha, pl., fr. Gr. ? folded,
   doubled; di- = di`s- twice + ? to fold, double up.]
   1. Anything consisting of two leaves. Especially:
      (a) (Roman Antiq.) A writing tablet consisting of two
          leaves of rigid material connected by hinges and
          shutting together so as to protect the writing within.
      (b) A picture or series of pictures painted on two tablets
          connected by hinges. See Triptych.
          [1913 Webster]

   2. A double catalogue, containing in one part the names of
      living, and in the other of deceased, ecclesiastics and
      benefactors of the church; a catalogue of saints.
      [1913 Webster]

WordNet (r) 3.0 (2006):

diptych
    n 1: a painting or carving (especially an altarpiece) on two
         panels (usually hinged like a book)