The Collaborative International Dictionary of English v.0.48:

Predella \Pre*del"la\, n. [It.]
   The step, or raised secondary part, of an altar; a
   superaltar; hence, in Italian painting, a band or frieze of
   several pictures running along the front of a superaltar, or
   forming a border or frame at the foot of an altarpiece.
   [1913 Webster]