The Collaborative International Dictionary of English v.0.48:

Triptych \Trip"tych\, n. [Gr. ? consisting of three layers or
   plates; ? (see Tri-) + ?, ?, a fold, layer.]
   Anything in three parts or leaves. Specifically:
   (a) A writing tablet in three parts, two of which fold over
       on the middle part.
       [1913 Webster]
   (b) A picture or altarpiece in three compartments.
       [1913 Webster]

WordNet (r) 3.0 (2006):

triptych
    n 1: art consisting of a painting or carving (especially an
         altarpiece) on three panels (usually hinged together)

Moby Thesaurus II by Grady Ward, 1.0:

117 Moby Thesaurus words for "triptych":
   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, clover, collage,
   color print, commonplace book, copy, court calendar, cyclorama,
   daub, daybook, desk calendar, deuce-ace, diary, diptych, docket,
   engagement book, engraving, fresco, icon, illumination,
   illustration, image, journal, leash, 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, set of three, shamrock, spiral notebook,
   stained glass window, stencil, still life, table, tableau, tablet,
   tapestry, tercet, ternary, ternion, terzetto, three, threesome,
   tierce, trefoil, trey, triad, trialogue, triangle, tricorn,
   trident, triennium, trihedron, trilogy, trimester, trine, trinity,
   trinomial, trio, triphthong, triple crown, triple threat, triplet,
   triplopy, tripod, trireme, triseme, triskelion, trisul,
   triumvirate, triunity, trivet, troika, wall painting, workbook,
   writing tablet, yearbook