PREDELLA. – In wood or marble, it is the wooden panel or marble slab, rectangular in shape and very elongated, often divided into various compartments, which forms the base of a polyptych. In the predella, scenes related to the images and figures depicted in the polyptych are generally represented. The predella may, however, also be found in paintings consisting of a single unit, especially if it is a large altarpiece, i.e., an "altarpiece."
Enciclopedia Cattolica