Anything in three parts or leaves.

A writing tablet in three parts, two of which fold over on the middle part.

A picture or altarpiece in three compartments.

three paneled painting hinged together so that the two smaller panels close over a square central panel, like the doors of a cupboard. In some instances, the three paintings were arranged to represent a progression from the past (left panel), to the present (central panel), to the future (right panel).