Source : Webster's Revised Unabridged Dictionary (1913)

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.
   (b) A picture or altarpiece in three compartments.

Source : WordNet®

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