Subgrade is a term that refers to the natural or prepared surface that serves as the foundation for a road, railroad, or similar construction. Synonyms for the word include base, foundation, pavement, bed, and base course. The subgrade is a critical component of any construction project as it provides support for the superstructure. It is typically made of compacted materials such as gravel, sand, or asphalt, and must be strong enough to withstand the weight of the traffic or load it will bear. Regardless of the term used, the subgrade is an essential feature of any infrastructure and must be carefully engineered to provide a stable foundation for the structure above it.