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]
predella, n.
1 an altar-step, or raised shelf at the back of an altar.
2 a painting or sculpture on this, or any picture forming an appendage to a larger one esp. beneath an altarpiece.