triptych

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A work consisting of three painted or carved panels that are hinged together.
2. n. A hinged writing tablet consisting of three leaves, used in ancient Rome.

Century Dictionary and Cyclopedia

1. n. A picture, carving, or other representation in three compartments side by side: most frequently used for an altar-piece. The central picture is usually complete in itself. The subsidiary designs on either side of it are smaller, and frequently correspond in size and shape to one half of the principal picture, to which they are joined by hinges so that they can be folded over and form a cover to it. The outsides of the folding parts or shutters have sometimes designs painted on them.
2. n. A series of writing-tablets, three in number, hinged or tied together. When used for spreading with wax, and writing with the stylus, the outer leaves were recessed for the wax on the inside only, the middle leaf on both sides. These are made of fir-wood, beech-wood, baked clay, ivory, and other material.

Wiktionary

1. n. A picture or series of pictures painted on three tablets connected by hinges.
2. n. A set of three se-tenant postage stamps that form a composite picture.

GNU Webster's 1913

1. n. A writing tablet in three parts, two of which fold over on the middle part.
2. n. A picture or altarpiece in three compartments.

WordNet 3.0

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