A reredos (/ -, -/, REER-dos, REER-ih-, RERR-ih-) is a large altarpiece, a screen, or decoration placed behind the altar in a church. It often includes religious images.
A reredos can be made of stone, wood, metal, ivory, or a combination of materials. The images may be painted, carved, gilded, composed of mosaics, and/or embedded with niches for statues. Sometimes a tapestry or another fabric such as silk or velvet is used.
The term reredos is sometimes confused with the term retable. While a reredos is generally placed on the floor behind an altar, a retable is placed either on the altar or immediately behind and attached to the altar. In French (and sometimes in English by confusing the terms), a reredos is called a retable; in Catalan a retaule, in Spanish a retablo, etc. Reredos is derived through Middle English from the 14th century Anglo-Norman areredos, which in turn is from arere behind +dos back, from Latin dorsum. The term referred generally to an open hearth of a fireplace or a screen placed behind a table. Used in the 14th and 15th centuries, reredos had become nearly obsolete until revived in the 19th century.
A reredos differs from a retable in its size and positioning; while a reredos typically rises from ground behind the altar, the retable is smaller and stands either on the back of the altar or on a pedestal behind it. Many altars have both a reredos and a retable." But this distinction may not always be observed. The retable may have become part of the reredos when an altar was moved away from the wall. For altars that are still against the wall, the retable often sits on top of the altar, at the back, particularly when there is no reredos (a dossal curtain or something similar is used instead). The retable may hold flowers and candlesticks.
The term reredos may also be used for similar structures, if elaborate, in secular architecture, for example very grand carved chimneypieces.