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.
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 is also where the altar cross, flowers and "office light" type candlesticks sit.