The Collaborative International Dictionary of English v.0.48:

Altarpiece \Al"tar*piece`\, n.
   The painting or piece of sculpture above and behind the
   altar; reredos.
   [1913 Webster]

WordNet (r) 3.0 (2006):

altarpiece
    n 1: a painted or carved screen placed above and behind an altar
         or communion table [syn: altarpiece, reredos]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "altarpiece":
   Communion table, abstract, abstraction, altar, altar carpet,
   altar desk, altar facing, altar of prothesis, altar rail,
   altar side, altar slab, altar stair, altar stone, ancona,
   block print, bomos, chancel table, collage, color print, copy,
   credence, cyclorama, daub, diptych, engraving, eschara, fresco,
   frontal, gradin, hestia, holy table, icon, illumination,
   illustration, image, likeness, mensal, miniature, missal stand,
   montage, mosaic, mural, panorama, photograph, picture, predella,
   print, prothesis, representation, reproduction, retable, retablo,
   rood altar, scrobis, stained glass window, stencil, still life,
   superaltar, tableau, tapestry, triptych, wall painting