reredos
Concise Oxford English Dictionary © 2008 Oxford University Press:
reredos /ˈrɪədɒs/
▶noun (pl. same) an ornamental screen at the back of an altar in a church.
– origin ME: from Anglo-Norman Fr., from OFr. areredos, from arere ‘behind’ + dos ‘back’.
'reredos' also found in these Oxford entries:

