Backed fill is a term commonly used in construction, referring to the material used to fill a space or gap behind a retaining structure. Some synonyms for backed fill include backfill, fill dirt, structural fill, embankment fill, and engineered fill. Backfill is the most commonly used synonym, and refers to any material used to refill an excavated area, such as soil, sand, or gravel. Structural fill and engineered fill are more specific terms, indicating that the material used has been designed to provide additional structural support, such as with the use of geosynthetics or soil stabilization additives. Embankment fill refers specifically to filling a gap in a roadway or other infrastructure.