An image-bearing structure set on the rear part of the altar, abutting the back of the altarblock, or set behind the altar in such a way as to be visually joined with the altar when viewed from a distance. It is also sometimes called a retable, following the medieval term retrotabulum [retabulum, retrotabularium]. The altarpiece was never officially prescribed by the Church, but it did perform a prescribed function alternatively carried out by a simple inscription on the altarblock: to declare to which saint or mystery the altar was dedicated. In fact, the altarpiece did more than merely identify the altar; its form and content evoked the mystery or personage whose cult was celebrated at the altar. This original and lasting function influenced the many forms taken by the altarpiece throughout its history.
Alexander Nagel