A decorative architectural screen or structure behind an altar, often carved in stone or wood and enriched with niches, saints, reliefs, tracery, or a tabernacle.
A reredos is an ornamental structure or screen rising behind an altar. It may be carved in stone or wood and can incorporate figures, niches, relief panels, pinnacles, tracery, tabernacles, and painted or gilded surfaces.
Etymology
From Anglo-French areredos, “behind the back,” related to rear + dos, “back.”
Sculptor Notes
Reredos design is inseparable from architecture. The sculpture must be legible from the nave and harmonize with wall, vault, altar width, sanctuary lighting, and vertical architectural divisions.
References
https://www.metmuseum.org/art/collection/search/475525