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)