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)

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "diptych":
   Domesday Book, abstract, abstraction, account book, address book,
   adversaria, album, altarpiece, annual, appointment calendar,
   appointment schedule, blankbook, block print, blotter, calendar,
   cashbook, catalog, classified catalog, collage, color print,
   commonplace book, copy, court calendar, cyclorama, daub, daybook,
   desk calendar, diary, docket, engagement book, engraving, fresco,
   icon, illumination, illustration, image, journal, ledger, likeness,
   log, logbook, loose-leaf notebook, memo book, memorandum book,
   memory book, miniature, montage, mosaic, mural, notebook, pad,
   panorama, petty cashbook, photograph, picture, pocket notebook,
   pocketbook, police blotter, print, representation, reproduction,
   scrapbook, scratch pad, spiral notebook, stained glass window,
   stencil, still life, table, tableau, tablet, tapestry, triptych,
   wall painting, workbook, writing tablet, yearbook