1620s, "hinged, two-leaved tablet of wood, ivory, etc., with waxed inner surfaces, used by the Greeks and Romans for writing with the style," from Latin diptycha (plural), from late Greek diptykha, neuter plural of diptykhos "double-folded, doubled," from di- "two" (see di- (1)) + ptykhe "fold," which is of uncertain etymology. In art, "a pair of pictures or carvings on two panels hinged together," by 1852.
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Definitions of triptych from WordNet
art consisting of a painting or carving (especially an altarpiece) on three panels (usually hinged together);